Er singt wie ein Vogel.

Here wie means like or as in the sense of in the same way as:

• Er singt wie ein Vogel. = He sings like a bird.

This is different from als, which is often used for:

• comparisons of difference:
  größer als = bigger than
• roles/jobs/identity:
  Er arbeitet als Lehrer. = He works as a teacher.

So in this sentence, wie is correct because it compares the manner of singing.

Because wie ein Vogel is not the direct object of the verb. It is part of a comparison.

• Er singt = He sings
• wie ein Vogel = like a bird

The noun after wie stays in the case that fits the comparison structure here, which is nominative:

• ein Vogel = nominative
• einen Vogel = accusative

If Vogel were a direct object, then accusative might be needed, but here it is not.

In German, all nouns are capitalized.

So:

• der Vogel = the bird
• ein Vogel = a bird

This is one of the most noticeable spelling differences from English.

Er is the subject pronoun and means he.

German subject pronouns include:

• ich = I
• du = you
• er = he
• sie = she / they
• es = it
• wir = we
• ihr = you all
• Sie = formal you

In Er singt wie ein Vogel, er is the person doing the action.

Singt is the 3rd person singular form of the verb singen (to sing).

The present-tense forms are:

• ich singe
• du singst
• er/sie/es singt
• wir singen
• ihr singt
• sie/Sie singen

Since the subject is er, the verb must be singt.

Yes. German is a verb-second language in main clauses, which means the finite verb usually comes in the second position.

• Er singt wie ein Vogel.

Here:

1. Er = first element
2. singt = second element

The comparison phrase wie ein Vogel comes after the verb.

You could also move another element to the front in a longer sentence, but then the verb would still stay second.

A simple English-friendly guide is:

• Er ≈ air
• singt ≈ zingt / singkt
• wie ≈ vee
• ein ≈ eye-n
• Vogel ≈ FOH-guhl

A few helpful points:

• w in German sounds like English v
• v in Vogel is pronounced like f
• ie is usually a long ee sound
• g at the end of singt is pronounced, though in connected speech it may sound light

In normal usage here, it means like a bird, not as a bird.

• Er singt wie ein Vogel. = He sings in the way a bird sings.

If you wanted to talk about someone acting in the role of a bird, German would normally make that clearer in another way, depending on context.

So for a learner, the safest reading is: comparison, not identity or role.

A natural negative version is:

• Er singt nicht wie ein Vogel. = He does not sing like a bird.

Here nicht negates the comparison or the verb phrase.

You would not normally use kein here, because kein negates a noun phrase in a different way, and this sentence does not mean He sings no bird.

Yes, but it would mean something more specific:

• wie ein Vogel = like a bird, any bird / birdlike
• wie der Vogel = like the bird, a particular bird already known from context

So:

• Er singt wie ein Vogel. = a general comparison
• Er singt wie der Vogel. = he sings like that specific bird

For the general idea, ein Vogel is the normal choice.

Yes, it is perfectly understandable and natural as a simple comparison.

Depending on context, Germans might also say:

• Er singt sehr schön. = He sings very beautifully.
• Er singt wunderschön. = He sings wonderfully.
• Er singt wie eine Nachtigall. = He sings like a nightingale.

That last one sounds more poetic, because a Nachtigall is known for beautiful singing. But Er singt wie ein Vogel is still a correct and normal sentence.

Yes. Wie can compare larger structures too.

For example:

• Er singt, wie sein Vater singt. = He sings the way his father sings.

In your sentence, though, wie is followed only by a noun phrase:

• wie ein Vogel

So this is a simple comparison, not a full subordinate clause.