Jag kan inte låta bli att tänka på vad hon sa i går kväll.

Låta bli att + infinitive is an idiomatic pattern in Swedish.

In this sentence, kan inte låta bli att tänka på means can’t help thinking about.

A more literal breakdown would be something like can’t leave off / can’t refrain from thinking about, but that literal sense is not how you should translate it in normal English.

So:

• låta bli att göra något = refrain from doing something
• inte kunna låta bli att göra något = not be able to help doing something

Because Swedish main clauses usually follow the V2 rule: the finite verb comes in second position.

Here the order is:

• Jag = subject
• kan = finite verb
• inte = sentence adverb

So Jag kan inte ... is the normal main-clause order.

Compare a subordinate clause, where the order changes:

• ... att jag inte kan låta bli ...

So English speakers often want to put inte before the verb, but in a Swedish main clause that is usually wrong.

Because the construction is låta bli att + infinitive.

So the full pattern is:

• låta bli att tänka
• låta bli att säga
• låta bli att skratta

The att links the idiom to the following verb. In this sentence, tänka stays in the infinitive because it comes after that construction.

Because tänka på means think about.

Swedish often needs a preposition where English does too, but the exact preposition has to be learned with the verb. Here the natural expression is:

• tänka på något = think about something
• tänka på någon = think about someone

If you just say tänka, it can mean think in a more general sense:

• Jag tänker. = I’m thinking.
• Jag tänker på henne. = I’m thinking about her.

So på is required here because the sentence names what the person is thinking about.

Here vad means what and introduces a clause meaning what she said.

So:

• vad hon sa = what she said

This whole clause acts like the object of tänka på.

A close alternative is det hon sa, which means what she said or more literally the thing she said. The difference is small in many contexts, but vad hon sa is very natural here.

Because this is an embedded question / subordinate content clause, not a direct question.

Compare:

• Vad sa hon? = What did she say?
• Jag tänker på vad hon sa. = I’m thinking about what she said.

In direct questions, Swedish usually has inversion:

• Vad sa hon?

But in embedded clauses, Swedish uses normal subject-verb order:

• vad hon sa

This is a very common pattern and an important one to learn.

Both sa and sade are past forms of säga.

• hon sa = she said
• hon sade = she said

Sa is very common in modern spoken and written Swedish.
Sade is also correct, but can sound a bit more formal, literary, or old-fashioned depending on context and speaker.

So hon sa is the most natural everyday choice for many learners to use.

Because i går kväll is a fixed time expression meaning yesterday evening / last night.

In expressions like this, Swedish usually does not use the definite form:

• i går kväll
• i morse
• i natt

So kväll here is not being used as a regular noun meaning the evening. It is part of a time adverbial expression.

That is why kvällen would sound wrong here.

I går is the traditional and very common spelling, and it is the safest one for learners to use.

You may also see igår in modern writing, but i går is still a very standard choice, especially in careful writing.

So if you are unsure, write:

• i går
• i går kväll

Because the sentence talks about two different times:

• kan inte låta bli = the current situation now
• hon sa = something that happened in the past

So the meaning is:

• right now, I can’t stop thinking
• about something she said last night

This mix of present and past is completely normal in both Swedish and English.

In this sentence, it most naturally goes with sa:

• vad hon sa i går kväll = what she said last night

Because i går kväll comes right after sa, most readers will understand it that way.

If you wanted to make it clearer that last night refers to the thinking instead, you would usually place the time expression differently, for example:

• I går kväll kunde jag inte låta bli att tänka på vad hon sa.

Yes, you could, and it would sound natural.

But there is a slight nuance:

• kan inte låta bli att tänka på = can’t help thinking about
• kan inte sluta tänka på = can’t stop thinking about

The first is a very common idiomatic way to express that something keeps coming back into your mind.
The second focuses a bit more on the idea that the thinking is ongoing and won’t stop.

In many everyday situations, both work well.