what Different between this :

i did'nt understant you !

i don't understant you !

and you can say : i don';t want to go my school
Hi Jorge,

Welcome to the Forum.

Capital letters are an important part of English writing. You need to say

When I was studying English yesterday, I didn't understand what you were saying.

Best wishes, Clive
Just a minor correction, understant is really understand.

didn't - Did is a past form, so when you say that you are talking about before the current time like: Yesterday I didn't understand what you were say.

don't - is the current form. When you are talking about the present. Like: I don't understand what you are saying.

Hopefully that helps,
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is this correct sentence :

when i was studying english yesterday , i did'nt understand what you are saying !
no it will be... "when i was studying english yesterday, I didn't understand what you were saying"
 Clive's reply was promoted to an answer.
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