Indefinite pronouns like someone, anyone, everybody are not a big problem at a practical level, not even in a foreign language, but they are sometimes a problem at a theoretical level. In other words everbody (!) knows that there is a difference between "He asked anybody" and "He asked somebody" and that it is not very useful to ask anybody if a precise anwer is needed. However understanding the differences between different indefinite pronouns are even a problem for native speakers. In this forum for instance A diferença entre «alguma coisa» e «qualquer
coisa Portuguese native speakers struggle with the difference between alguma coisa, something, and qualquer coisa, anything. The problem is not that they don't know how to use it, the problem is that they can't explain to other people what is the difference.
Lets start with an obvious difference. Algum can be adjective or pronoun and has to agree in gender and number with the noun it refers to or with the noun it stands for. Qualquer can only be used as an adjective and is invariable. (There are very few invariable adjectives. Qualquer is one of these rare adjectives.) Alguém is always a pronoun and is invariable as well.
Algum is inflected as
The flexion is not the problem. The problem is to understand the difference between algum + noun and qualquer + noun.
The difference is that qualquer + noun stresses the arbitrarity with which an element of a group can be the executor (Anybody can do it that, this is not difficult) or the goal (You can ask anybody, anybody knows that) of an action. This is not the case with algum + noun (You better ask somebody who knows it.) If the arbitrarity is an irrelevant aspect qualquer + noun (anybody / anything ) can be substituted by algum + noun ( somebody / something). In this case the only relevant aspect is the fact that the refered person / thing is unknown.
Do you want to say anything?
Do you want to say something?
However if the arbitrarity is the central message anything can't be substituted by something.
1) Anything will be helpful.
2) Something will be helpful.
In 1) there are several options. If someone or somebody learn a language, anything related to language learning, watch youtube video, listen to audio books, speaking with native speakers, take classe, study a grammar book etc. is helpful. The sentence "Something will be helpful" possibly makes sense in some contexts, but it is a little bit difficult to imagine a context where it makes sense.
The Collins online dictionary translates qualquer / quaisquer with any (qualquer pessoa = any person, quaisquer livros = any books) and algum with some (alguns homens = some men). This is correct (more or less). However it is useful to understand that qualquer stresses the arbitrarity, algum / alguém the fact that something is unknown. Depending on the context that doesn't make any difference, but in other contexts it makes a big difference. (As already said, we are discussing a theoretical problem. For whatever reason nobody has a problem with that in practice, what is actually astonishing, because if one thinks about it, it is difficult.)
The same is true for the variants of algum and qualquer like alguém (somebody) and qualquer pessoa (anybody). The following sentences are both correct, but the meaning is completely different.
qualquer pessoa alguém
1) Mais que qualquer pessoa que eu conheço.
More than anybody I know.
2) Mais que alguém que eu conheço.
More than somebody I know.
In 1) he compares every single person he knows with someone, "He eats more than anybody I know". In 2) he compares someone he has in mind with another person.
All the following sentences are correct as well, but don't mean the same thing.
qualquer pessoa cada um alguém alguma
1) Perguntou a qualquer pessoa.
He asked just anybody.
2) Perguntou a cada um.
He asked everybody.
3) Perguntou a alguém.
He asked somebody.
4) Perguntou a alguma pessoa.
He asked some person.
The main assertion of 1) is that he asked the first person he encountered in the street. Implicetly there is the message that he should have asked someone who knows something about the issue. In the case 2) he asked every single person, one after the other. That can be useful. If a decision is to be taken in a team, it is better to ask every single member for his / her opinion. In the case 3) and 4) he asked someone unknown. The main assertion is, that this person is unknwon. There is no statement whether it was the right person or not or whether he should have asked someone else.
The same problem, the problem that in some contexts an indefinite pronoun can be substituted by another one and in other contexts not, occurs as well in the case of "seja quem for" / "seja que for", "whoever" / "whatever" and Someone / something. (For is a conjuntivo do futuro, see 12.2.5.) The meaning of these sentences is the same, in this context a substitution is possible.
Something stung me.
Whatever it has been, it stung me.
In both cases he doesn't know who or what stung him, but the fact that it is unknown is not the main assertion. In the following case the situation is completely different.
1) Somebody who founds it should call.
2) Whoever founds it should call.
We all know that 1) is absurd, the question is why. Somebody is used if the executor or the goal of an action is unknown, "Somebody knocks at the door". However this somebody exists. We can't use somebody in a context where it is not even sure that the executor or the goal exists. Whoever means, that there is potentially an executor, although it is not sure and that is what we need in this context. It is not sure that someone founds it. It is only a possibility. It is a strange phenomenon that nobody in any language has any difficulties in using whoever correctly, but everybody would need some time to answer if asked in which context it is used. It seems that the human brain knows much more than we know.
In English there is a difference between each and every. Normally it is said that the difference is that every is used if the single elements are considered as a group, " Every beat of my heart tears me further apart", and each if the single elements are considered isolated, "Each of them could be the culprit". This author would say however that the difference in meaning is not the crucial point. Each belongs to idiomatic expression, "each of them" or can be used as a synonym for both, "in both hands" <=> in each hand. It may be true that there is a preference for each if we focus on the single element, "He gave each child an apple" <=> "He gave every child an apple" and perhaps this preference was stronger in the past, but this author would say that this difference has vanished a little bit. In any case each as well as everything are translated with cada / todos - todas.
each (each day) = todos / todas (noun in plural = todos o dias), cada (noun in singular = cada dia)
every (noun in singular = every day) / all (noun in plural = all days) = todos / todas (noun in plural = todos o dias), cada (noun in singular = cada dia)
any (any day) = qualquer (qualquer dia)
Some languages, for instance german, doesn't distinguish between "He can come every day" and "He can come any day". This however is an important difference. If he can come any day he can come monday or tuesday or wednesday etc. If he can come every day he can come monday and tuesday and wednesday etc.. This distinction is made very clearly in Portuguese.
Sometimes any is used in English in the meaning of every / all: He fulfilled any of her wishes. <=> He fulfilled all her wishes. <=> He fullfilled every of her wishes. It is actually true that if he fullfilled whatever she wished, he fullfilled all, every and any of her wishes. However in Portuguese there is a preference for cada in this case. Qualquer is used to underline the arbitrarity.
cada every, each adjectiv
Pode-se ver cada linha e cada ponto.
You can see every line and and every point.
Considero cada pormenor, avalio cada possibilidade.
I look at every detail, I think about every possibility.
Procurem em cada vagão, falem com cada passageiro.
Search all the waggons and and speak with all passengers.
cada um noun
Depende das necessidades de cada um.
It depends on the needs of every single person.
Então decidi seguir cada um deles.
That's why I decided to follow each of them.
Cumprimos cada um dos seus mandamentos.
We have executed any of his commands.
qualquer pessoa jeder x-beliebige
Tenho os meus sentimentos como qualquer pessoa.
As anybody else I have feelings.
Fico tão excitado como qualquer pessoa.
I am as delighted as anyobdy else.
Então fiz o que faria qualquer pessoa responsável.
I did what anybody with a sens of responsibility would do.
Qualquer + noun can be used as a noun as well, in this case it is qualquer um.
The above mentioned aspects, arbitrarity, everybody can be the executor or the goal of an action, group or single element, don't play any role in the case of alguém. Alguém is used if the executor or the goal of an action is unknwon.
alguém somebody / someone
O nome de alguém apareceu no computador.
The name of someone appeared on the computer screen.
Tens de conhecer alguém que conhece alguém.
You have to know somebody who knows somebody.
Arranje alguém com os conhecimentos necessários.
Find someone with the required skills.
Furthermore we have some. Some is the plural of somebody / something. If we form the same sentence with different indefinite pronouns we see the difference. Every indefinite pronouns underlines a different aspect, otherwise we wouldn't need such a lot of them. However sometimes the meaning does not only change, but the whole sentence becomes meaningless.
1) Some are discouraged, others see the crisis as a chance.
2) ~Everybody is discouraged, others see the crisis as a chance.
3) ~All are discouraged, others see the crisis as a chance.
4) ~Whoever is discouraged, others see the crises as a chance.
5) Somebody is discouraged, others see the crisis as a chance.
Some is alguns in Portuguese.
Alguns não entenderão a minha decisão.
Some won't understand my decision.
Por que algumas pessoas cantam melhor do que as outras?