τἰ and accent rules

[Alacritas]

Χαίρετε!

I'm in the beginning stages of learning Greek at the moment, and I've come across something that confuses me a bit. I've tried to find an answer on Google, but am having trouble finding any results.

In Athenaze, I keep seeing τί with the acute accent preserved, whereas I was under the impression that acutes change to graves when they precede another word.

Ex. :

"τί λέγεις, ὦ θύγατερ; [...]"

"τί ποιεῖτε, ὦ δοῦλοι;"

So is it the case that all interrogatives function this way? Just "τί"?

Any help in my understanding this matter would be greatly appreciated. Thank you.

[Grochojad]

"τί" is an exception to the general rule of acutes becoming graves and so keeps it's accent unchanged.