Learn Italian: Non sono mica stupido, ho già capito tutto. - It's not like I'm stupid, I have already understood everything.

mica is used to emphasize a negation, similar to saying 'at all' or 'it is not like' in English. While non sono stupido just means 'I am not stupid', adding mica makes it stronger and slightly defensive.

Can I use mica without non in this sentence?

Yes, but only if you change the word order! When mica comes after the verb, you must use non before the verb, exactly like non sono mica. If you move mica to the very front of the sentence, the non is dropped, making it Mica sono stupido.

Why is già placed right in the middle of ho capito?

In compound tenses like the passato prossimo, short adverbs of time like già (already), ancora (yet), and mai (ever or never) usually go right between the helping verb (ho) and the past participle (capito).

Why does tutto come at the very end of the sentence?

tutto means 'everything' and acts as the direct object here. Just like in English, the direct object typically follows the verb it belongs to. So, ho già capito tutto follows the standard natural word order.