"What are you looking at?"
Translation:Na co se díváš?
It strongly competes for that position, but you have to first identify what "first position" actually is. You seem to have wanted "Na se co díváš?", and that tells me you think the first position is automatically the first word in the sentence. Pasting here from the discussion linked by VladaFu:
But what is the “first position”? In Czech, it is not necessarily just one word. For example, we could [...] have
- Jeho krásná žena ho už nemiluje.
- Jeho krásná žena, pro kterou by modré z nebe snesl, ho už nemiluje.
The key there is that we need to be “done” with the first coherent piece of the sentence, even if it takes a subordinate clause to get there. So
- ???Jeho krásná ho žena už nemiluje.
is wrong because (“jeho krásná“ what?) we clearly were not done with that first piece. Identifying the end of the first complete constituent of the sentence should not feel entirely strange to an English speaker. “His beautiful does not love him wife anymore.” or “His does not love him anymore beautiful wife.” should feel ill-formed as well. The subject was not finished when we rushed in with the predicate.
In your sentence ''se'' is not in the second place, that is the issue. ''na co'' is the first group so ''se'' has to follow. Then you can say ''na co se divaš'' or 'na co se ty divaš'' but not what you wrote. ''se'' in second position is fixxed. You could change the order and say ''Ty se divaš na co'' also, as long as ''se'' is second.
I just want to emphasize that the most important words in Mairn4's post may be "'na co' is the FIRST GROUP so 'se' has to follow." The "first position" may be held by a single word -- or it may be held by a group of words closely linked to one another in terms of their meaning and their role in the sentence.
Actually, "na co ty se díváš?" sounds completely natural, too - even a little more natural than "na co se ty díváš?". If I wanted to add the pronoun "ty", I would place it just there: "Na co ty se díváš?", unless I wanted to stress it even more strongly: "Na co se díváš ty?".
I usually can explain these things, but here I'm at a loss as to why "ty" can occupy the second position ahead of "se".
Also note that adding "ty" (at any position) makes a contrast with someone else looking at something - e.g. as a follow up to "Já se dívám na strop." (I'm looking at the ceiling.) or "František se dívá na kočku." (F. is looking at a cat.) - and what is it that YOU are looking at?
To is here a particle that makes the sentence stronger. It has no direct translation. Compare with: Now, what you are looking at? You also would not translate now in a literal way as "teď, nyní", it also mostly just stresses something. But it is certainly not the same as the "to".