Im Tarifvertrag stehen die Zuschläge.

Im is a contraction of in dem.

• in = in
• dem = the (dative masculine/neuter)

So im Tarifvertrag literally means in the collective agreement / in the pay agreement.

In German, contractions like im are very common:

• im = in dem
• am = an dem
• zum = zu dem

Here, Tarifvertrag is masculine, so dem is the correct form in the dative.

Because after in with a location meaning, German uses the dative case.

The base noun is:

• der Tarifvertrag = the collective agreement / wage agreement

In the dative singular, masculine nouns usually become:

• dem Tarifvertrag

So:

• in dem Tarifvertrag
• contracted: im Tarifvertrag

There is no extra -s here because Tarifvertrag is not one of the masculine/neuter nouns that add -(e)s in the dative.

Yes, stehen literally means to stand, but in German it is also often used in the sense of:

• to be written
• to be stated
• to appear in a text

So Im Tarifvertrag stehen die Zuschläge means something like:

• The bonus payments are stated in the collective agreement
• The bonus rates are listed in the agreement

This is a very common use of stehen with written information:

• Das steht im Buch. = That is written in the book.
• Was steht in der E-Mail? = What does the email say?

Because the subject is die Zuschläge, and that is plural.

• der Zuschlag = the bonus / surcharge / supplement
• die Zuschläge = the bonuses / surcharges / supplements

German verbs must agree with the subject:

• der Zuschlag steht = the bonus is stated
• die Zuschläge stehen = the bonuses are stated

So stehen is plural because die Zuschläge is plural.

Die Zuschläge is the subject of the sentence.

Even though it comes after the verb, it is still the subject. German does not always put the subject before the verb.

Here is the structure:

• Im Tarifvertrag = first position
• stehen = verb in second position
• die Zuschläge = subject

So the sentence is not saying that the agreement is doing something to the bonuses. It is saying that the bonuses / extra payments are stated in the agreement.

That version is also correct.

German main clauses follow the verb-second rule, which means the finite verb must be in the second position. But the first position can be filled by different elements.

So both are possible:

• Die Zuschläge stehen im Tarifvertrag.
• Im Tarifvertrag stehen die Zuschläge.

The difference is mainly emphasis:

• Die Zuschläge ... puts focus on the bonuses
• Im Tarifvertrag ... puts focus on where they are stated

English is usually stricter about word order, but German is more flexible as long as the verb stays in second position.

Tarifvertrag usually means a collective agreement or collective bargaining agreement.

It is a formal agreement, often between:

• employers or an employers’ association
• and a trade union

It can regulate things like:

• pay
• working hours
• overtime
• bonuses
• holiday entitlement

So in this sentence, it suggests that the Zuschläge are officially laid out in that agreement.

Zuschläge can mean things like:

• supplements
• bonuses
• extra payments
• surcharges
• premium rates

The exact translation depends on context.

In a work or pay context, Zuschläge often refers to extra pay for things like:

• night work
• weekend work
• holiday work
• overtime

So in this sentence, it likely means extra wage rates or bonus payments specified in the collective agreement.

Here it is plural.

German die can mean:

• the for a feminine singular noun
• the for any plural noun

In this sentence, it is plural because:

• the noun is Zuschläge
• the verb is stehen (plural)

So:

• die Zuschläge = the bonuses / supplements

Not naturally in this sentence if you want the meaning are written/stated in.

• Die Zuschläge sind im Tarifvertrag sounds incomplete or odd in most contexts.
• Die Zuschläge stehen im Tarifvertrag is the normal idiomatic wording.

German often uses stehen for information contained in a text, document, email, law, contract, or agreement.

So if the idea is the bonuses are listed/stated in the agreement, then stehen is the right choice.