Hon kunde inte låta bli att le när morfar började sjunga.

Låta bli att + infinitive is a fixed expression meaning to refrain from doing something or to leave something undone.

With negation, inte låta bli att, it usually means can’t help doing something.

So:

• låta bli att le = refrain from smiling
• inte låta bli att le = not be able to stop oneself from smiling / can’t help smiling

This is one of the most useful idiomatic patterns in Swedish.

Kunde is the past tense of kan.

• kan = can
• kunde = could

The whole sentence is talking about a past situation, and later you also have började = began, which is also past tense. So the tense is consistent:

• Hon kunde inte...
• morfar började...

Also, in this idiom, kunde inte låta bli is best understood as couldn’t help rather than simple ability.

Because this is a main clause, and in Swedish main clauses the finite verb normally comes in the second position. The negation inte usually comes after that finite verb.

So:

• Hon kunde inte låta bli...

Here:

• Hon = subject
• kunde = finite verb
• inte = negation

This is very typical Swedish word order.

Compare:

• Hon ler inte. = She is not smiling.
• Hon kunde inte le. = She could not smile.

Because after låta bli att, Swedish uses the infinitive form of the next verb.

• le = infinitive, to smile
• ler = present tense, smiles / is smiling

So:

• låta bli att le = refrain from smiling
• not låta bli att ler

This is similar to English using to + verb after certain expressions.

After börja, Swedish often uses a bare infinitive, especially in natural everyday language.

So:

• började sjunga = began to sing

You may also sometimes see började att sjunga, but började sjunga is very common and usually sounds more natural.

So in this sentence:

• morfar började sjunga = grandfather began singing / began to sing

Here när means when and introduces a time clause.

• när morfar började sjunga = when grandpa started singing

It is not a question word here. It is functioning as a conjunction that links the two parts of the sentence.

Very often, när can be translated as:

• when
• sometimes as, depending on context

Morfar specifically means mother’s father, so maternal grandfather.

Swedish distinguishes between the two grandfathers:

• morfar = mother’s father
• farfar = father’s father

Likewise for grandmothers:

• mormor = mother’s mother
• farmor = father’s mother

English usually just says grandfather, but Swedish is more specific.

You should mainly learn it as an idiomatic expression.

Literally, the parts are:

• låta = let
• bli = remain / become / stay, depending on context

But låta bli att as a whole does not work well word-for-word in English. The real meaning is:

• refrain from
• avoid
• with inte: can’t help

So it is much better to memorize the full pattern:

• låta bli att + infinitive
• inte kunna låta bli att + infinitive

For example:

• Jag lät bli att ringa. = I refrained from calling.
• Jag kunde inte låta bli att skratta. = I couldn’t help laughing.

No. The standard expression is låta bli att + infinitive.

So you need all of this structure:

• låta bli att le

If you remove bli, the idiom is broken and the sentence does not mean the same thing.

So:

• correct: Hon kunde inte låta bli att le
• not correct here: Hon kunde inte låta att le

Yes, absolutely. It is a normal expression that works with different people and tenses.

Examples:

• Jag låter bli att äta godis. = I refrain from eating candy.
• Han lät bli att svara. = He refrained from answering.
• Vi kunde inte låta bli att skratta. = We couldn’t help laughing.
• De har låtit bli att kommentera. = They have refrained from commenting.

So the important thing to remember is the pattern, not just this one sentence:

• låta bli att + infinitive
• often with inte kunna for can’t help doing something

Yes. Log simply means smiled.

But kunde inte låta bli att le adds the idea that the smile was hard to resist or happened almost automatically.

So the nuance is something like:

• Hon log. = She smiled.
• Hon kunde inte låta bli att le. = She couldn’t help smiling.

The longer expression makes the reaction feel more spontaneous and emotionally natural.