Jag försöker behålla lugnet när bussen är sen.

Because lugnet is the definite form of lugn. In Swedish, behålla lugnet is a very common expression meaning keep calm or keep your cool.

Using the definite form here sounds natural and idiomatic. It refers to the calm state as something you are trying not to lose.

You can also say behålla sitt lugn, which means almost the same thing, but behålla lugnet is especially common.

Försöker is the present tense of försöka, which means to try or to attempt.

So:

• jag försöker = I try / I am trying

In this sentence, it describes an ongoing effort: the speaker is trying to stay calm.

After försöka, Swedish can use either:

• försöka + infinitive
• försöka att + infinitive

So both of these are possible:

• Jag försöker behålla lugnet
• Jag försöker att behålla lugnet

In everyday Swedish, leaving out att is very common, so the original sentence sounds completely natural.

Behålla means keep, retain, or hold on to.

In this sentence, it does not mean physically keeping an object. It means keeping a mental or emotional state:

• behålla lugnet = keep calm
• more literally: retain the calm

So it is about not losing your composure.

När means when, while om usually means if.

Here, när bussen är sen means when the bus is late. It suggests a situation that happens or is being talked about as a real event.

If you used om, it would sound more conditional:

• om bussen är sen = if the bus is late

So:

• när = when this happens
• om = if this happens

Both can be possible in some contexts, but när fits well if the speaker means a real or recurring situation.

Bussen means the bus. The definite form is used because it refers to a specific bus in the situation, usually the one the speaker is waiting for or talking about.

Compare:

• en buss = a bus
• bussen = the bus

So the sentence is talking about a particular bus, not buses in general.

Yes. Here sen means late.

So:

• bussen är sen = the bus is late

This is a very natural everyday way to say it. Swedish also has försenad, which means delayed, and that can also work:

• bussen är försenad = the bus is delayed

The difference is mostly style:

• sen = more everyday
• försenad = a little more formal or technical

Because bussen is a singular en-word, and the common form of the adjective is sen.

Swedish adjectives change depending on the noun. For example:

• en buss är sen
• ett tåg är sent
• bussarna är sena

So in your sentence:

• bussen is singular and common gender
• therefore sen is the correct form

Because när bussen är sen is a subordinate clause.

In Swedish subordinate clauses, the subject usually comes before the verb:

• när bussen är sen

Not:

• när är bussen sen

The second version looks like a direct question: When is the bus late?

So the original sentence follows normal Swedish subordinate clause word order.

Yes. If the subordinate clause comes first, the main clause usually has inversion.

So you can say:

• När bussen är sen försöker jag behålla lugnet.

Notice the order in the main clause:

• försöker jag
• not jag försöker

That happens because the sentence begins with something other than the subject.

They are similar, but not exactly identical.

• behålla lugnet = keep calm / keep your cool
• hålla sig lugn = stay calm

Behålla lugnet often suggests that something stressful is happening and you are trying not to lose composure.

Hålla sig lugn focuses more on remaining calm as a state.

In many situations, both work, but behålla lugnet sounds especially idiomatic in this kind of sentence.

Yes. När can refer to:

• a specific occasion: when the bus is late today
• a general/repeated situation: whenever the bus is late

So this sentence could mean either:

• I try to keep calm when the bus is late
• I try to keep calm whenever the bus is late

The exact meaning depends on context.