"The bear is an animal."
Translation:Ayı bir hayvandır.
I'm certainly not an expert, but these seem to be two forms of the same suffix. From what I can tell, the letter D is used following vowels or voiced consonants, while T is used following unvoiced consonants. The vowel is determined by the standard 4-way harmony pattern.
After weeks of confusion, a native speaker cleared something up for me. I wish this section had a clue/info card on this.
When dealing with "is" just follow this table: I am: ben -im You are: sen -sin He/she/it is: o -dır We are: biz -iz Y'all are: siz -siniz They are: onlar -dırlar
Of course you need to follow vowel harmony, consonant transformation, and there are exceptions, but that table has helped me a lot and I'm surprised it's not provided to learners. I'm guessing that's because it's too formal and spoken Turkish leaves out a lot of formalities.
Just my 2 cents, but i think a language so different than English as Turkish should be taught formally (book form) so as to help build a structural understanding.