Learn Turkish: O köpekler büyük.

This is because of 2-way vowel harmony! You look at the last vowel in the word köpek, which is 'e'. Since 'e' is a front vowel, it pairs with the -ler suffix. If the last vowel had been a back vowel (like a, ı, o, or u), you would use -lar.

The translation says 'those dogs'. Why do we use O ('that') instead of Onlar ('those')?

In Turkish, when a pointing word like O (that/those) or Bu (this/these) comes right before a noun to describe it, it never takes a plural suffix. Since the noun köpekler is already plural, O stays in its base form. You would only use Onlar if it was standing alone as a pronoun, for example if you were just saying 'They are big' (Onlar büyük).

Why doesn't the word for big (büyük) get a plural suffix too?

When the subject of the sentence already has a plural suffix (like köpekler), the adjective at the end of the sentence doesn't need one. Saying O köpekler büyük is the standard, most natural way to say 'Those dogs are big.'

Where is the word for 'are' in this sentence?

Turkish doesn't have a separate word for 'to be' (am/is/are). Instead, it uses personal suffixes attached to the final word. However, for the third person ('he/she/it' and often 'they'), the suffix is usually completely invisible! So, O köpekler büyük literally translates to 'Those dogs big', but it forms a complete, grammatically correct sentence.