Learn Romanian: Sunt de acord cu tine.

Romanian doesn't have a single verb for "to agree". Instead, it uses the phrase a fi de acord, which literally means "to be of agreement". Because of this, you use the verb a fi (to be) matched to the speaker: sunt de acord means "I am of agreement".

Does the word acord change if multiple people agree?

No, acord is a noun meaning "agreement" and stays exactly the same no matter who is speaking. Only the verb changes. For example, "we agree" is suntem de acord.

Why is it cu tine instead of cu tu?

After prepositions like cu (with), Romanian requires the object form of the pronoun (the accusative case). The object form of tu (you) is tine.

How would I say "I don't agree with you"?

To make this negative, you just place nu (not) directly before the verb. The sentence becomes Nu sunt de acord cu tine.