How to Say "you understood." in Spanish
The Spanish word for “you understood.” is “entendiste” — A1 level. This is a very common word in everyday Spanish.

Examples
Te lo dije despacio, ¿entendiste?
I told you slowly, did you understand?
Cuando me explicaste el problema, entendiste que no era mi culpa.
When you explained the problem to me, you understood it wasn't my fault.
No, no entendiste la diferencia entre los dos sistemas.
No, you didn't understand the difference between the two systems.
The Informal Past 'You'
This form is used when speaking to a single person you know well (using 'tú'). It describes a completed action in the past—the moment understanding either happened or didn't happen.
Preterite Tense Ending
For '-er' and '-ir' verbs in the simple past (Preterite), the 'tú' form always ends in '-iste' (comiste, viviste, entendiste).
Using the Present Tense by Mistake
Mistake: “¿Entiendes?”
Correction: Use 'entendiste' when asking about something that was explained moments ago. '¿Entiendes?' (Do you understand, generally?) is for checking current knowledge; '¿Entendiste?' (Did you get that one thing I just said?) is for checking past comprehension.
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