"This is a bird."
Translation:Bu bir kuş.
You can translate "bu bir kuştur". If the word ending with an hard consonant (Turkish hard consonants: p,ç,t,k,s,ş,h,f) the suffixs first consonant is mutating "t" in this ex. -dır mutating -tır, -dir mutating -tir, -dur mutating -tur, -dür mutating -tür... if you asked when we use -tır or -tur or -tür, it is about vowel harmony....
Nope, "bunu" is in the accusative case and subjects can never be in the accusative.
because there is no Turkish equivalent to "a", "bir" just means "one", this is unnecessary because plurals in turkish are at the end of the word- one bird= kus, multiple birds= kuslar, if you don'd include the "bir" in the sentence, you still know its one singular bird because there is no "-lar" at the end of "kus". "bu kus"=this (is) bird, "bu bir kus"= this (is) one bird
Someone correct me if I'm wrong, but i think it's because turkish doesn't have articles, and bir litteraly means "one", so you can omit it except when you want to emphasise that for example 'he eats 1 apple'. And bir cannot be omitted when you have a word+adjective, for example güzel bir kuş, büyük bir elma etc. :)