Jag blir frustrerad när planen ändras i sista minuten.

Blir is the present tense of bli, which often means become or get.

So Jag blir frustrerad is literally I get frustrated.

That fits this sentence well because it describes a reaction that happens when something changes.
If you said Jag är frustrerad, that would mean I am frustrated, which describes a state more than the change into that state.

Both can work in some contexts, but:

• Jag blir frustrerad när ... = I get frustrated when ...
• Jag är frustrerad när ... = I am frustrated when ...

The version with blir sounds especially natural when talking about something that triggers the feeling.

Because frustrerad describes the person who feels the emotion, while frustrerande describes the thing that causes the emotion.

• Jag är frustrerad = I am frustrated
• Det är frustrerande = It is frustrating

In your sentence, jag is the one feeling frustration, so Swedish uses frustrerad.

A useful pattern is:

• -ad / -ed type meaning: the person feels it
• -ande / -ing type meaning: the thing causes it

So:

• Jag blir frustrerad
• Att planerna ändras är frustrerande

Here när means when.

It introduces a time clause:

• när planen ändras = when the plan is changed / when the plan changes

It tells you under what circumstances the speaker gets frustrated.

Be careful not to confuse this with när? as a direct question word:

• När kommer du? = When are you coming?
• Jag blir frustrerad när ... = I get frustrated when ...

Same word, different function.

Because planen is the definite form, meaning the plan.

In Swedish, definiteness is often added to the end of the noun:

• en plan = a plan
• planen = the plan

So:

• när planen ändras = when the plan is changed

If you said när en plan ändras, that would mean when a plan is changed, which is more general and less tied to a specific plan.

Ändras is the -s passive form of ändra.

• ändrar = changes / is changing something
• ändras = is changed

So:

• Någon ändrar planen = Someone changes the plan
• Planen ändras = The plan is changed

In your sentence, the focus is on the fact that the plan changes, not on who changes it. Swedish very often uses the -s passive for this.

In natural English, you might translate it either as:

• when the plan is changed
• when the plan changes

Even though the Swedish form is passive, English may sound more natural with either version depending on context.

Because after när, you have a subordinate clause, and Swedish word order changes in subordinate clauses.

In a main clause, Swedish usually follows V2 word order, where the verb comes in second position. But in subordinate clauses, the subject usually comes before the verb.

So:

• Main clause style: Planen ändras i sista minuten
• Subordinate clause after när: när planen ändras i sista minuten

That is why planen comes before ändras here.

I sista minuten means at the last minute.

It is an idiomatic expression used when something happens very late, just before it is too late or just before something begins.

Examples:

• Han kom i sista minuten. = He arrived at the last minute.
• De ändrade bokningen i sista minuten. = They changed the booking at the last minute.

So in your sentence, it means the plan gets changed very late, which is what causes frustration.

In this fixed expression, Swedish normally says i sista minuten without den.

This is a common idiomatic time phrase, just like some English expressions drop words in set phrases.

So you should learn i sista minuten as a chunk meaning at the last minute.

Even though den sista minuten can exist in other contexts and literally means the last minute, the idiomatic expression for this situation is:

• i sista minuten

No. Planen here is singular: the plan.

Forms:

• en plan = a plan
• planen = the plan
• planer = plans
• planerna = the plans

So if you wanted when the plans change, you would say:

• när planerna ändras

This is a very useful pattern to memorize for en nouns.

Yes. That would mean I get frustrated when the plans change at the last minute.

The difference is just singular vs. plural:

• planen = the plan
• planerna = the plans

Use planen if you mean one specific plan.
Use planerna if you mean arrangements more generally or several plans.

Both are completely natural depending on what you want to say.