Mücken stechen manchmal, wenn wir im Garten sitzen.

In German, a plural noun can be used without an article when talking about things in general.

So Mücken stechen manchmal means mosquitoes sometimes bite/sting in a general sense, not the mosquitoes in a specific situation.

Compare:

• Mücken stechen manchmal. = Mosquitoes sometimes bite.
• Die Mücken stechen manchmal. = The mosquitoes sometimes bite.

The version without die sounds more like a general statement.

The singular is die Mücke.

So:

• die Mücke = one mosquito
• die Mücken = mosquitoes

This is one of those nouns where the singular and plural article are both die, so you tell the difference from the noun form:

• singular: Mücke
• plural: Mücken

Literally, stechen often means to sting or to prick/pierce. With insects like mosquitoes, German commonly uses stechen, even though in English we usually say bite.

So in this sentence:

• natural German: Mücken stechen
• natural English: mosquitoes bite

A learner should remember that German and English do not always choose the same verb here.

Manchmal is an adverb meaning sometimes, and German often places time/frequency adverbs in the middle field of the sentence, after the finite verb in a main clause.

So:

• Mücken stechen manchmal ...

This is a very natural position.

You could also move manchmal for emphasis, but the basic version here is normal and idiomatic.

For example:

• Manchmal stechen Mücken, wenn wir im Garten sitzen.
  This is possible too, but it puts more focus on sometimes.

Because wenn wir im Garten sitzen is a subordinate clause, and in German subordinate clauses are normally separated from the main clause by a comma.

So the sentence has:

• main clause: Mücken stechen manchmal
• subordinate clause: wenn wir im Garten sitzen

German punctuation is stricter here than English punctuation often is.

Wenn can mean when in the sense of:

• whenever
• if
• repeated or general situations

In this sentence, it introduces a situation that happens sometimes/repeatedly, not a one-time event.

So wenn wir im Garten sitzen means something like:

• when we are sitting in the garden
• whenever we sit in the garden

A common comparison:

• wenn = when/whenever in repeated or general situations
• als = when in a one-time event in the past

Example:

• Wenn wir im Garten sitzen, stechen Mücken manchmal. = Whenever we sit in the garden, mosquitoes sometimes bite.
• Als wir gestern im Garten saßen ... = When we were sitting in the garden yesterday ...

It is at the end of the subordinate clause.

After conjunctions like wenn, German sends the conjugated verb to the end of the clause:

• wenn wir im Garten sitzen

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

Compare:

• main clause: Wir sitzen im Garten.
• subordinate clause: ..., wenn wir im Garten sitzen.

So the verb sitzen moves from second position to the end because of wenn.

Because wenn creates a subordinate clause, and subordinate clauses follow different word order from main clauses.

Main clause word order:

• Wir sitzen im Garten.

Subordinate clause word order:

• ..., wenn wir im Garten sitzen.

So after wenn, the subject wir comes before other sentence elements, and the verb sitzen goes to the end.

Im is the contraction of in dem.

So:

• im Garten = in dem Garten

This is extremely common in German:

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

Here, Garten is masculine:

• der Garten

That is why dem is used.

Because this sentence describes location, not movement.

German uses:

• in + dative for location: in dem Garten / im Garten
• in + accusative for movement into something: in den Garten

So:

• Wir sitzen im Garten. = We are sitting in the garden.
• Wir gehen in den Garten. = We are going into the garden.

Since sitting describes being in a place, the dative is correct.

Yes. German often allows that.

You could say:

• Wenn wir im Garten sitzen, stechen Mücken manchmal.

That is also correct. When the subordinate clause comes first, the main clause still follows the normal rule that the finite verb comes in second position:

• Wenn wir im Garten sitzen, stechen ...

Notice that stechen comes immediately after the comma because the first clause takes up position 1.

In German, the simple present often covers both ideas:

• we sit
• we are sitting

So wir sitzen can mean:

• we sit
• we are sitting

In this sentence, English would most naturally use are sitting, but German does not need a separate progressive form the way English often does.

It sounds like a general/habitual statement, mainly because of:

• Mücken without an article
• manchmal
• wenn

Together they suggest something like a repeated experience:

• mosquitoes sometimes bite when we sit in the garden

If you wanted a more specific one-time situation, the sentence would probably look different, often with more context or different time expressions.