"Lawyers drink a lot of tea."
Translation:Avukatlar çok çay içer.
Well, they are both perfectly acceptable. "içer" is the third person singular form and "içerler" is the third person plural form. The thing is, with the third person plural you can use either the verb for the third person singular (in this case içer) or the verb for the third person plural (in this case içerler). My thought is that there might be an instance where using içerler instead of içer might remove some ambiguity, but really either is perfectly fine. :)