Because it's çorbaYI with the accusative ending, and (to a first approximation) only definite objects get marked with the accusative.
So if you see an accusative ending, it'll be definite: either "the" or some other determiner such as "that" or "my" will be appropriate in the English translation.
- they are drinking = içiyorlar
- they drink = içerler
Simple present ("They are drinking"):
- Verb stem + i (4-way vowel harmony) + yor + personal suffix
- iç + i + yor + lar
Note: if verb stem ends in a vowel, it will be REPLACED by -i (with vowel harmony). For example: ödemek (to pay) = ödüyorlar (they are paying)
Aorist Tense ("They drink"):
These are a little more complicated because it depends on how many syllables are in the verb stem.
- 1 syllable verb stem + ar (2-way harmony) + personal suffix
- iç + er + ler
- 2 or more syllable verb stem + ir (4-way harmony) + personal suffix
- öğren + ir + ler ("they learn")
Note: if the verb stem ends in a vowel, we just add -r + personal suffix. Ex. ödemek (to pay) = öderler (they pay)