Questions & Answers about Er trinkt ein Bier.
Because it is the first word of the sentence. In German, the first word is capitalized just like in English.
Normally, the pronoun is written er, not Er:
- Er trinkt ein Bier.
- Ich glaube, dass er ein Bier trinkt.
A useful extra note: the formal Sie is capitalized even in the middle of a sentence, but er is not.
In a normal German main clause, the finite verb usually comes in position 2.
So in:
- Er = position 1
- trinkt = position 2
- ein Bier = the rest of the sentence
This is one of the most important German word-order rules. English also often has subject + verb order, but German is stricter about the verb being in second position in main clauses.
Because trinkt is the form that agrees with er.
The verb is trinken = to drink.
Its present-tense forms are:
- ich trinke
- du trinkst
- er/sie/es trinkt
- wir trinken
- ihr trinkt
- sie/Sie trinken
Since the subject is er = he, you need trinkt.