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.

Ein Bier is in the accusative case because it is the direct object of the verb trinkt.

He is doing the drinking, and the beer is the thing being drunk.

So:

• Er = subject
• trinkt = verb
• ein Bier = direct object, therefore accusative

Because Bier is a neuter noun: das Bier.

The indefinite article ein changes depending on gender and case. In the accusative singular:

• masculine: einen
• feminine: eine
• neuter: ein

Since Bier is neuter, you say:

• ein Bier

not:

• eine Bier

Because all German nouns are capitalized.

So:

• der Mann
• die Frau
• das Bier

This is different from English, where common nouns are usually lowercase.

It can mean either, depending on context.

In many cases, ein is just the indefinite article:

• a beer

But it can also emphasize the number:

• one beer

Without special context, Er trinkt ein Bier is usually understood as He is drinking a beer.

Yes. That would also be correct, but the meaning changes slightly.

• Er trinkt ein Bier. = He is drinking a beer / one beer
• Er trinkt Bier. = He drinks beer / He is drinking beer

Without the article, Bier sounds more general, like a substance or category. With ein, it refers more to a specific serving or instance.

It can mean either one.

German often uses the simple present tense where English might use either:

• He drinks a beer
• He is drinking a beer

The exact meaning depends on context. German does not usually need a separate progressive form like English is drinking.

Yes, but the verb trinkt still has to stay in second position in a normal main clause.

For example:

• Er trinkt ein Bier.
• Ein Bier trinkt er.

The second version is possible, but it puts special emphasis on ein Bier and sounds less neutral. The most normal basic order is:

• subject + verb + object

A few useful pronunciation points:

• tr at the beginning is pronounced together.
• i in trinkt is a short vowel.
• nk is pronounced as you would expect.
• The final t is clearly pronounced.

Also, the verb trinken has a different vowel sound from Bier:

• trinkt → short i
• Bier → long ie sound

And in German, ie is usually pronounced like a long ee sound.

Because for neuter singular, the indefinite article is the same in the nominative and accusative:

• nominative: ein Bier
• accusative: ein Bier

So the case is not obvious from ein alone. You know it is accusative because of the sentence structure and the role of the noun: it is the thing being drunk.

This is different from masculine nouns, where the accusative changes more clearly:

• der Wein → einen Wein in the accusative