Het is waarschijnlijk langer als het verkeer drukker is dan normaal.

In this sentence, het works a lot like English it in It is probably longer...

It may refer to something understood from context, such as:

• a trip
• a route
• a delay
• the travel time

But Dutch also often uses het as a general placeholder subject, just as English does with it.

Waarschijnlijk means probably.

Its position here is very normal:

• Het is waarschijnlijk langer

In Dutch main clauses, an adverb like waarschijnlijk often comes after the finite verb and before the adjective or other main information.

You could also say:

• Waarschijnlijk is het langer

That version is also correct, but it shifts the emphasis slightly toward probably.

Langer is the comparative form of lang, so it means longer.

Dutch forms many comparatives by adding -er:

• lang → langer
• druk → drukker

Here, langer can describe something that takes more time or has greater length, depending on the context.

Good question. Here, als does not belong to langer.

It introduces a subordinate clause:

• als het verkeer drukker is dan normaal

Here als means if or when, not as in a comparison.

So the structure is:

• Het is waarschijnlijk langer
• als het verkeer drukker is dan normaal

If Dutch were directly comparing with langer, standard Dutch would normally use dan, for example:

• langer dan gisteren

So in this sentence:

• als = if/when
• dan normaal belongs to drukker, not to langer

Because als introduces a subordinate clause.

In Dutch subordinate clauses, the finite verb usually moves to the end:

• main clause: Het verkeer is drukker dan normaal
• subordinate clause: als het verkeer drukker is dan normaal

This verb-final pattern is one of the most important Dutch word-order rules to get used to.

Drukker is the comparative of druk.

With verkeer, druk means busy or heavy:

• druk verkeer = heavy traffic
• het verkeer is druk = the traffic is busy/heavy
• het verkeer is drukker = the traffic is busier/heavier

So het verkeer drukker is dan normaal means that traffic is heavier than usual.

Dan normaal means than normal or than usual.

It completes the comparison started by drukker:

• drukker dan normaal = busier than normal

This is a very common Dutch pattern:

• warmer dan normaal = warmer than normal
• rustiger dan normaal = quieter than normal
• later dan normaal = later than normal

So here normaal acts as the standard of comparison.

In this sentence, it most naturally describes drukker:

• het verkeer drukker is dan normaal = the traffic is busier than normal

That is the most natural reading because dan normaal comes right after drukker is inside the subordinate clause.

If you wanted to make longer than normal clearer, Dutch would usually phrase it differently, for example:

• Het is waarschijnlijk langer dan normaal als het verkeer drukker is.
• or more naturally for time: Het duurt waarschijnlijk langer dan normaal als het verkeer drukker is.

Yes. In Dutch, als can often mean either if or when, depending on context.

Here it suggests a general condition or situation:

• if traffic is heavier than normal
• when traffic is heavier than normal

Both are possible ways to understand it.

If you want to be more clearly time-based, Dutch can also use wanneer:

• wanneer het verkeer drukker is dan normaal

If you want a more formal if, Dutch can use indien, but that would sound more formal than this sentence needs.

It is understandable, but if the meaning is about time, Dutch often prefers duren:

• Het duurt waarschijnlijk langer als het verkeer drukker is dan normaal.

That is often more idiomatic than Het is waarschijnlijk langer when you mean something takes longer.

So:

• Het is langer = can work, depending on context
• Het duurt langer = often the more natural choice for duration

A learner might very often hear the duren version in real Dutch.