It sais in there, this for 3rd, person singular
3) The suffix -DIr is used to clarify any ambiguity, emphasize, or state facts. This both follows 4-way vowel harmony and has consonant harmony; ‘d’ changes to ‘t’ after the following consonants (p ç t k s ş h).
Does that mean it does exist, but you only uses it if there's is going to be ambiguity?
not even especially for ambiguity problems, we use it really rarely. -DIr is used mainly for facts (Kan kırmızıdır-blood is red). Using it in other kind of sentences is really weird. We also use it for making a guess but we don't really explain this in the course. For example:
- Who do you think is the new girlfriend of Gordon?
- Bence Ayşe'dir (I think it is Ayşe)
- Hayır bence Elif'tir (No, I think it is Elif)
in general, avoid using it a lot except for "encyclopedic knowledge"