Am Vormittag arbeite ich.

German main clauses usually follow the verb-second rule: the finite verb must be in the second position.

So in:

Am Vormittag arbeite ich.

• Am Vormittag = first position
• arbeite = second position
• ich = comes after the verb

If you start with the time expression, the subject usually moves after the verb.

You can also say:

Ich arbeite am Vormittag.

That is also correct. The difference is mostly in emphasis:

• Am Vormittag arbeite ich. = emphasis on when
• Ich arbeite am Vormittag. = more neutral

Am is a contraction of an dem.

• an = a preposition
• dem = dative form of der

So:

• an dem Vormittag → am Vormittag

In expressions of time, German often uses am with parts of the day:

• am Morgen
• am Nachmittag
• am Abend
• am Vormittag

So here am Vormittag means in the morning / during the morning, more literally on the morning part of the day.

Both can relate to the morning, but they are not always exactly the same.

• der Morgen = morning in a general sense
• der Vormittag = the part of the day before noon, often especially the later morning

So:

• am Morgen = in the morning
• am Vormittag = in the forenoon / during the morning before noon

In many everyday situations, they can be similar, but Vormittag often sounds a bit more specific.

Because Vormittag is a noun, and in German all nouns are capitalized.

Examples:

• der Vormittag
• die Arbeit
• das Haus

So even in the middle of a sentence, German nouns begin with a capital letter.

arbeite is the first-person singular form of the verb arbeiten in the present tense.

Conjugation:

• ich arbeite = I work
• du arbeitest
• er/sie/es arbeitet
• wir arbeiten
• ihr arbeitet
• sie/Sie arbeiten

Since the subject is ich, the correct verb form is arbeite.

Yes, absolutely.

Both are correct:

• Am Vormittag arbeite ich.
• Ich arbeite am Vormittag.

The first one puts more focus on the time.
The second one is the more straightforward English-like order.

German word order is flexible, but the verb still stays in second position in a normal main clause.

Yes. In am Vormittag, the noun is in the dative because the preposition an is being used in a time expression, and here it takes the dative.

• an dem Vormittag = dative
• contracted: am Vormittag

You do not need to memorize a deep literal explanation every time; it is best to learn am Vormittag as a common fixed time phrase.

German present tense can cover several ideas that English separates.

Am Vormittag arbeite ich. can mean things like:

• I work in the morning.
• I am working in the morning.
• I’ll be working in the morning.

The exact meaning depends on the context.

German often uses the simple present where English might use:

• simple present
• present progressive
• even a future meaning

Yes, but the meaning changes slightly.

• am Vormittag = on a particular morning period / in the morning
• vormittags = in the mornings, usually or regularly

Examples:

• Am Vormittag arbeite ich. = I work in the morning / this morning period
• Vormittags arbeite ich. = I work in the mornings

So vormittags often sounds more habitual.

A rough pronunciation guide is:

ahm FOR-mi-tahk AR-bye-te ikh

A few notes:

• Vormittag has stress on the first syllable: VOR
• arbeite sounds like AR-bye-te
• ich has the soft German ch sound, not an English k or sh

If that ich sound is hard, try making a very soft hiss after an ee sound:

• eeh + soft hiss → ich

Yes, it is completely natural.

It is a simple, correct German sentence, and it sounds normal.
A native speaker might choose between:

• Am Vormittag arbeite ich.
• Ich arbeite am Vormittag.

depending on what they want to emphasize, but both are standard and natural.