The phonem [ĩ] is represented by the graphems -im / -in. (That doesn't exist in French either. The graphem in / em is not prounced as a nasal i in French.)

IPA presentation graphem example
[ĩ]im sim (yes), impedir (to impede) ,
[ĩ]in inseto (insect)

If one didn't pay attention perhaps one would not realise that it is nasale vowel. However there is a difference to a "normal" i.

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