Der Gast kommt herein.

Because Gast is a masculine noun in German. Its basic article is der.

• der Gast = the guest
• plural: die Gäste = the guests

In this sentence, der Gast is the subject, so it stays in the nominative case: der Gast.

Kommt is the 3rd person singular form of kommen.

The verb kommen means to come.

Here is the present-tense pattern:

• ich komme
• du kommst
• er/sie/es kommt
• wir kommen
• ihr kommt
• sie/Sie kommen

Because der Gast is a singular he/she/it-style subject grammatically, German uses kommt.

Because this is a separable verb: hereinkommen.

In many German main clauses, separable verbs split into:

• the conjugated verb in position 2
• the prefix at the end

So:

• full verb: hereinkommen
• in the sentence: kommt ... herein

That is why German says:

• Der Gast kommt herein.

not:

• Der Gast hereinkommt.

in a normal main clause.

The full verb is hereinkommen.

You can think of it like this:

• kommen = to come
• herein = in / inside, toward the speaker’s location
• hereinkommen = to come in

So herein is not the main verb by itself here. It is the separable part of hereinkommen.

Herein adds the idea of coming in / entering inward, often with a sense like in here.

A useful contrast:

• hinein = in there / into there, usually movement away from the speaker’s perspective
• herein = in here, usually movement toward the speaker’s perspective

In everyday German, this distinction is often not stressed very strongly, but it is good to know.

Often, yes, in everyday meaning they are very close.

• hereinkommen = to come in
• reinkommen = to come in

Rein is more informal/colloquial and very common in speech.

So you may hear:

• Der Gast kommt herein.
• Der Gast kommt rein.

Both can mean basically the same thing, but herein can sound a bit more explicit or slightly more formal/written.

Because Gast is the subject in the nominative singular.

German nouns do not always take extra endings in the nominative singular. The article often shows the case clearly:

• der Gast = nominative
• den Gast = accusative

So in this sentence, Gast stays just Gast.

Yes. That would also be correct.

The difference is:

• Der Gast kommt herein. = the guest comes in
• Ein Gast kommt herein. = a guest comes in

So the article changes depending on whether the guest is specific/known (der) or not yet identified (ein).

Because German main clauses usually follow the V2 rule: the finite verb goes in the second position.

In this sentence:

1. Der Gast
2. kommt
3. herein

Even if something else comes first, the verb still stays second:

• Jetzt kommt der Gast herein.
• Dann kommt der Gast herein.

This is one of the most important word-order patterns in German.

Yes, absolutely. That is very natural in spoken German.

• herein = a bit more explicit, sometimes slightly more formal or written
• rein = very common and conversational

So if you are speaking casually, Der Gast kommt rein sounds normal.

Then the separable verb stays together as hereinkommen.

Examples:

• infinitive: Der Gast will hereinkommen.
• subordinate clause: ..., weil der Gast hereinkommt.

So:

• main clause: Der Gast kommt herein.
• verb together: hereinkommen / hereinkommt

This is a very useful pattern to learn with separable verbs.

It is pronounced roughly like heh-RINE.

A rough English-friendly guide:

• her- is not like English her
• ein sounds like the English word mine without the m

So the whole word sounds approximately like:

• heh-RINE

The stress is usually on the second part: herein.

German present tense often covers both:

• simple present: The guest comes in
• present progressive: The guest is coming in

So Der Gast kommt herein can describe:

• a general present-tense event in narration
• something happening right now, depending on context

German usually does not need a separate form like English is coming.