Wenn wir jetzt nicht losgehen, verpassen wir den Zug.

Wenn introduces a subordinate clause and here it means if.

So Wenn wir jetzt nicht losgehen = If we don’t leave now.

A useful note: wenn can also mean when in other contexts, but in this sentence it is clearly conditional, so if is the right idea.

Because wenn is a subordinating conjunction, and subordinating conjunctions send the conjugated verb to the end of the clause.

Compare:

• Main clause: Wir gehen jetzt los.
• Subordinate clause: Wenn wir jetzt nicht losgehen, ...

Also, losgehen is a separable verb. In a normal main clause, it splits:

• Wir gehen los.

But in a subordinate clause, it stays together:

• ..., wenn wir losgehen

So both things are happening here:

• wenn pushes the verb to the end
• the separable verb losgehen stays unsplit

German main clauses follow the verb-second rule. The conjugated verb must come in the second position.

Here, the entire wenn-clause comes first:

• Wenn wir jetzt nicht losgehen, = position 1

So the verb of the main clause must come next:

• verpassen = position 2
• wir comes after that

That is why German says:

• Wenn wir jetzt nicht losgehen, verpassen wir den Zug.

and not:

• Wenn wir jetzt nicht losgehen, wir verpassen den Zug.

German very often uses the present tense to talk about the future, especially when the time is already clear from context.

Here, the meaning is about the future, but German does not need a special future form:

• Wenn wir jetzt nicht losgehen, verpassen wir den Zug.

English often does something similar too:

• If we don’t leave now, we miss the train can be understood, though English more often says we’ll miss the train.

German could also use werden, but it would usually sound less natural here:

• ..., werden wir den Zug verpassen.

That is grammatical, but the simple present is more common.

Losgehen is a separable verb.

Its basic idea here is to set off / to leave / to get going.

In a main clause, it separates:

• Wir gehen jetzt los.

In a subordinate clause, it stays together:

• wenn wir jetzt nicht losgehen

This is a very common pattern with separable verbs:

• anfangen → Wir fangen an / weil wir anfangen
• mitkommen → Kommst du mit? / ob du mitkommst

Because nicht usually comes before the part it is negating.

Here it negates the action losgehen:

• jetzt nicht losgehen = not leave now

So the sentence means: the problem is that we do not leave now.

If you moved nicht, the emphasis could change. For example:

• wenn wir nicht jetzt losgehen would sound more like if we leave not now (but at some other time)

That is much less natural here. The original version is the normal one.

Because verpassen takes a direct object in the accusative case.

The noun is der Zug in the dictionary form, but after verpassen it becomes accusative:

• der Zug → nominative
• den Zug → accusative

So:

• Wir verpassen den Zug.

Other examples:

• Ich verpasse den Bus.
• Sie verpassen den Anfang des Films.

Yes. In German, a subordinate clause is normally separated from the main clause by a comma.

So this comma is required:

• Wenn wir jetzt nicht losgehen, verpassen wir den Zug.

This is different from English, where comma use can be a bit more flexible in some cases. In German, with a wenn-clause like this, the comma is standard and expected.

Yes. You could say:

• Wenn wir jetzt nicht losgehen, dann verpassen wir den Zug.

That is also correct.

The word dann means then, and it can make the if-then relationship more explicit. But it is often omitted in German, just as in English:

• If we don’t leave now, we miss the train.
• If we don’t leave now, then we miss the train.

Both are fine.

Both can mean if, but there is a small nuance.

• wenn is the normal, neutral choice
• falls can sound a bit more like in case or a bit more hypothetical

So:

• Wenn wir jetzt nicht losgehen, verpassen wir den Zug.
  normal and natural

• Falls wir jetzt nicht losgehen, verpassen wir den Zug.
  also possible, but slightly less neutral in tone

In everyday German, wenn is the most common choice here.

Yes, wenn can mean both if and when.

You tell from context.

Here the sentence is clearly conditional:

• If we don’t leave now, we’ll miss the train.

So wenn means if.

A few useful comparisons:

• Wenn es regnet, bleiben wir zu Hause.
  If/When it rains, we stay home.
  This can be general or conditional depending on context.

• Als ich klein war, ...
  When I was little, ...
  Use als for a one-time event or period in the past.

• Wann kommt der Zug?
  When does the train come?
  Use wann in direct or indirect questions.

Yes, absolutely.

That version is also correct:

• Wir verpassen den Zug, wenn wir jetzt nicht losgehen.

The difference is mainly in emphasis and word order:

• Wenn wir jetzt nicht losgehen, verpassen wir den Zug.
  starts with the condition

• Wir verpassen den Zug, wenn wir jetzt nicht losgehen.
  starts with the result

In the second version, the main clause comes first, so normal word order stays:

• Wir verpassen ...

There is no inversion after the main clause because the wenn-clause comes second.

This is a normal, real, likely condition, so ordinary present tense is exactly right.

The sentence means something like:

• this is a real possibility
• if we don’t leave now, the consequence will happen

German uses normal present tense for that:

• Wenn wir jetzt nicht losgehen, verpassen wir den Zug.

If the condition were unreal or contrary to fact, German would often use Konjunktiv II, for example:

• Wenn wir jetzt nicht losgingen, würden wir den Zug verpassen.

But that sounds more hypothetical and less natural for the immediate real-life situation in your example.