Если на мосту авария, троллейбус может застрять в пробке на полчаса.

Because Если на мосту авария is a subordinate if-clause, and Russian normally separates it from the main clause with a comma.

So the structure is:

Если на мосту авария, | троллейбус может застрять в пробке на полчаса.

This is more regular in Russian than in English: when the если clause comes first, you use a comma.

Russian says на мосту for on the bridge.

There are two things happening here:

1. на is the normal preposition for being on a bridge.
2. мост has a special location form мосту after в and на when it means actual physical location.

So:

• на мосту = on the bridge

For modern standard Russian, на мосту is the normal form.

It is a special locative-type form used after в and на with some nouns when talking about location.

A learner often expects regular prepositional-case endings, but some common masculine nouns have this -у / -ю form in location expressions:

• в лесу = in the forest
• в шкафу = in the cupboard
• на мосту = on the bridge

So although it may look unusual, it is a standard pattern.

Because Russian normally omits to be in the present tense.

Literally, the phrase is something like:

If on the bridge [there is] an accident

Russian does this all the time:

• Он врач = He is a doctor
• Дома никого = There is nobody at home
• На мосту авария = There is an accident on the bridge

So the verb is understood, not spoken.

You do not know from the noun alone, because Russian has no articles.

So троллейбус can mean:

• the trolleybus
• a trolleybus

Context decides. In a general statement like this, English might use either depending on the situation:

• the trolleybus if a specific trolleybus is understood
• a trolleybus if the statement is more general

This is a very common difference between Russian and English.

Может is the 3rd person singular of мочь = to be able / can.

Here it expresses possibility, not permission. So it means something like:

• can
• may
• might

In this sentence, троллейбус может застрять means the trolleybus can/might get stuck.

Because застрять is perfective, and here Russian is talking about a possible single result: the trolleybus may end up getting stuck.

• застрять = to get stuck, to become stuck once
• застревать = to get stuck repeatedly / to be in the process of getting stuck / habitual imperfective idea

After может, Russian often uses a perfective infinitive when talking about a possible future event or result:

• может прийти = may come
• может сломаться = may break
• может застрять = may get stuck

Grammatically, может is present tense, but the whole phrase often refers to a possible future outcome.

So:

• может застрять literally uses present-tense может
• but in meaning it is often can/might get stuck

That is very normal in Russian. A present-tense modal verb plus a perfective infinitive often points to a future possibility.

Because the fixed expression is застрять в пробке = to get stuck in traffic / in a traffic jam.

Here:

• в means in
• пробке is the prepositional form of пробка

Literally, пробка also means cork, but in everyday Russian it very commonly means traffic jam.

So:

• в пробке = in a traffic jam
• застрять в пробке = to get stuck in traffic

На полчаса means for half an hour.

Russian often uses на + time expression to show the duration or extent of something, especially when something lasts for a certain period:

• уйти на час = to leave for an hour
• опоздать на десять минут = to be ten minutes late
• застрять на полчаса = to get stuck for half an hour

So here:

троллейбус может застрять в пробке на полчаса
= the trolleybus may get stuck in traffic for half an hour

Yes. Russian word order is flexible.

For example, you could also say:

Троллейбус может застрять в пробке на полчаса, если на мосту авария.

That means essentially the same thing.

The version with Если... first puts the condition first and sounds very natural. The other version puts the main idea first and the condition after it. Both are correct.