Я не только не взял зонт, но и забыл шарф дома.

Because they do two different jobs:

• the first не is part of the fixed expression не только ... но и ... = not only ... but also ...
• the second не directly negates the verb взял = did not take

So не только не взял literally means not only did not take.

This is completely normal in Russian. It is not a “double negative” in the English sense; it is just the combination of:

• the structure не только
• plus a negative verb не взял

Не только ... но и ... is a very common paired construction meaning not only ... but also ....

In this sentence, it links two actions:

• не взял зонт = didn’t take an umbrella
• забыл шарф дома = forgot/left the scarf at home

So the overall structure is:

• Я не только не взял зонт, но и забыл шарф дома.
• I not only didn’t take an umbrella, but also forgot my scarf at home.

Russian often uses this structure very much like English, but the negation stays exactly where it belongs:

• не только не взял
• но и забыл

Взял is the perfective past tense of взять. Here it shows a single completed action: the speaker failed to take the umbrella on that particular occasion.

Russian aspect matters a lot:

• взял = took / did take / took once, completed
• брал = was taking / used to take / tried taking / took repeatedly, depending on context

In this sentence, the speaker is talking about one specific situation, so взял is the natural choice.

Compare:

• Я не взял зонт. = I didn’t take an umbrella.
• Я не брал зонт. = more like I wasn’t taking the umbrella / I didn’t take the umbrella (in a broader, less event-focused sense)

For this kind of “I forgot to bring it” situation, не взял is the normal form.

For the same reason: забыл is perfective, showing a single completed event.

• забыл = forgot
• забывал = used to forget / was forgetting / forgot repeatedly

Here the speaker means one concrete incident:

• I forgot my scarf at home

So забыл is the right choice.

If you said забывал, it would suggest repeated behavior or a different kind of context, for example:

• Я часто забывал шарф дома. = I often forgot my scarf at home.

Both зонт and шарф are in the accusative case because they are direct objects of the verbs:

• взял зонт = took an umbrella
• забыл шарф = forgot a scarf / forgot the scarf

However, both nouns are:

• masculine
• singular
• inanimate

For masculine inanimate nouns, the accusative looks the same as the nominative. That is why you see:

• зонт instead of some visibly different form
• шарф instead of some visibly different form

So the case is accusative, even though the form does not change.

Russian does not have articles.

So зонт can mean:

• an umbrella
• the umbrella

and шарф can mean:

• a scarf
• the scarf
• sometimes even my scarf, if context makes that clear

Russian leaves this to context. In this sentence, English would usually supply something natural such as:

• an umbrella
• my scarf

but Russian does not need separate words for that.

Here дома is an adverb meaning at home.

So:

• забыл шарф дома = forgot/left the scarf at home

This is a very common Russian expression:

• Я дома. = I’m at home.
• Он остался дома. = He stayed at home.
• Я забыл ключи дома. = I left my keys at home.

By contrast, в доме means in the house/building, with more emphasis on the physical building itself.

So in everyday speech, when talking about one’s home, дома is usually the natural choice.

In English, the most natural interpretation is often left the scarf at home.

Russian забыть can mean not only “to forget mentally,” but also “to forget something somewhere,” which often implies leaving it behind.

So:

• Я забыл шарф дома literally = I forgot the scarf at home
• natural English sense = I left my scarf at home

Both ideas are closely connected, and Russian uses забыл for this very naturally.

Yes, Russian word order is flexible, although not completely free.

The sentence as given is very natural:

• Я не только не взял зонт, но и забыл шарф дома.

You might also hear:

• Я не только не взял зонт, но и шарф забыл дома.

This still means the same thing, but the position of шарф gives it a bit more emphasis.

What usually stays important is the paired structure:

• не только ... но и ...

That pairing should remain clear, even if some internal word order changes.

Yes, Russian can often omit subject pronouns when the verb already shows the person.

So you could say:

• Не только не взял зонт, но и забыл шарф дома.

and it would still be understood as I from context, especially in conversation.

But Я is often included:

• for clarity
• for emphasis
• because it sounds natural at the start of a sentence

So both are possible, but Я is perfectly normal here.