Om en knapp lossnar, kan jag sy fast den själv.

Om here means if and introduces a condition.

So Om en knapp lossnar ... means If a button comes loose ...

Be careful: om can also mean about in other sentences, but in this sentence it is clearly the conditional if.

Lossnar is the present tense of lossna.

Swedish often uses the present tense in if-clauses when talking about a possible future situation, just like English does:

• Om det regnar, stannar jag hemma. = If it rains, I stay home / I’ll stay home.
• Om en knapp lossnar, kan jag ... = If a button comes loose, I can ...

So even though the situation may happen later, the present tense is normal here.

Lossna means to come loose, to loosen, or to become detached.

It is an intransitive verb, which means the thing itself comes loose:

• Knappen lossnar. = The button comes loose.

A useful comparison is lossa, which is usually transitive:

• Jag lossar knappen. = I loosen the button.

So:

• lossna = come loose
• lossa = loosen something

This is because Swedish follows the V2 rule in main clauses: the finite verb normally comes in the second position.

The first part, Om en knapp lossnar, is a subordinate clause placed first. After that, the main clause begins, and its finite verb must come first within that clause:

• Om en knapp lossnar, kan jag sy fast den själv.

Here the order is:

1. Om en knapp lossnar = first position
2. kan = finite verb in second position
3. jag = subject

If you put the main clause first, you get normal subject-verb order:

• Jag kan sy fast den själv om en knapp lossnar.

Both are correct.

Because kan is a modal verb, and in Swedish modal verbs are followed by the infinitive without att.

So:

• kan sy
• ska sy
• vill sy
• måste sy

Not:

• kan att sy

In this sentence:

• kan = can
• sy = sew

So kan sy = can sew

No. Here fast does not mean quickly.

In this sentence, sy fast is a verb phrase meaning to sew on, to sew back on, or to sew so that something is attached firmly.

So:

• sy = sew
• fast = attached / fixed in place

Together:

• sy fast en knapp = sew a button on
• sy fast den = sew it on

This is a very natural way to talk about attaching something by sewing.

Because knapp is an en-word in Swedish:

• en knapp

Pronouns must match the grammatical gender of the noun:

• en-words → den
• ett-words → det

So:

• en knapp → den

If the noun were an ett-word, you would use det instead.

Själv here means myself or by myself.

So kan jag sy fast den själv means that I can sew it on myself, without needing someone else to do it.

It adds emphasis. Without själv, the sentence would still make sense:

• Om en knapp lossnar, kan jag sy fast den.

Adding själv highlights independence or personal ability:

• ... kan jag sy fast den själv.

En knapp means a button, so it is indefinite.

That fits well when you are speaking generally: if any button comes loose, I can sew it back on.

If you were talking about one specific button already known from the context, you might use the definite form:

• Om knappen lossnar, kan jag sy fast den själv.

So the choice depends on whether the button is being introduced generally or referred to specifically.

Yes, absolutely.

• Om en knapp lossnar, kan jag sy fast den själv.
• Jag kan sy fast den själv om en knapp lossnar.

Both are correct and mean essentially the same thing.

The difference is mainly structure and emphasis:

• starting with Om en knapp lossnar foregrounds the condition
• starting with Jag kan ... foregrounds what I can do

This is also a good way to see the word-order rule:

• after an initial om-clause: kan jag
• when the main clause starts the sentence: jag kan