"Dívala ses?"

[gSee4]

Why is the second person here? And how to say Did she watch?

[Paul440375]

Normally "jsi" indicates the second person in past tense but here you use "ses" which substitutes "jsi se" ("jsi se" is colloquial). Third person would be "dívala se".

[vytah]

What words can the -s ending be attached to? So far I've seen it with past participles and now with "se".

[JanLyko]

When using a reflexive verb in the second-person singular past form and in conditional, the auxiliary verb být (“to be”) is replaced with just -s appended to the reflexive pronoun se, si. The full form “jsi se”, “jsi si” is proscribed as hypercorrect.

Then some verbs and pronouns

Ty jsi tam byl? = Tys tam byl? = Byls tam?

[linguistenator]

Is the alternative "Dívala se jsi"?

[JanLyko]

Almost right. "Dívala jsi se".

[JanisaChatte]

But why doesn't "se" come second ?

[standelf]

https://en.wikipedia.org/wiki/Czech_word_order#Clitics

[Tibro3]

And why would, "You watched?" not be correct?

[ion1122]

DL generally does not recognize punctuation. (This is true for all their language programs.)

So your sentence would be interpreted as a statement instead of a question. In DL you cannot make a question just by sticking a "?" at the end of a sentence.