Возьми зонтик, а то промокнешь.

Both can mean take, but they come from different aspects:

• взять → возьми = perfective
• брать → бери = imperfective

In this sentence, возьми is natural because the speaker means take it now / make sure you take it as a single completed action.

• Возьми зонтик = Take an umbrella
• Бери зонтик can also be possible, but it often feels more like go ahead and take the umbrella, take it (I’m offering it), or a more general/repeated instruction depending on context.

So возьми fits the idea of one specific action before going out.

Зонтик is in the accusative singular because it is the direct object of возьми.

The noun is:

• зонтик = umbrella

For inanimate masculine nouns, the accusative singular is usually the same as the nominative singular. So:

• nominative: зонтик
• accusative: зонтик

That is why the form does not change here.

А то is a very common conversational expression meaning:

• or else
• otherwise
• if not

In this sentence, it introduces a warning or consequence:

• Возьми зонтик, а то промокнешь.
• Take an umbrella, otherwise you’ll get soaked.

It is slightly informal and very common in everyday speech.

Промокнешь is the 2nd person singular future form of the perfective verb промокнуть.

• промокнуть = to get soaked / become wet

Perfective verbs do not have a true present tense. Their “present-looking” forms actually refer to the future:

• я промокну = I’ll get soaked
• ты промокнешь = you’ll get soaked

That makes sense here, because the soaking would happen later, after the person goes outside.

Russian often leaves out subject pronouns when they are obvious from the verb ending.

Here:

• возьми clearly shows a command to you (singular)
• промокнешь clearly means you will get soaked

So Russian does not need ты.

You could say Ты возьми зонтик..., but that usually adds emphasis, contrast, or a particular tone. In neutral everyday speech, leaving ты out is more natural.

Yes, they are related.

• мокрый = wet
• промокнуть = to get soaked / become thoroughly wet

The prefix про- often adds the idea of becoming wet through and through. So промокнуть is stronger than simply being a little wet.

In this sentence, it suggests getting properly soaked in the rain.

Because а то connects two clauses:

• Возьми зонтик
• а то промокнешь

In Russian, this kind of clause connection is separated by a comma. It works much like English punctuation in:

• Take an umbrella, or else you’ll get soaked.

So the comma is standard here.

Both mean umbrella.

• зонт = umbrella
• зонтик = literally a diminutive form, but in modern Russian it is extremely common and often just sounds natural in everyday speech

In many contexts, зонтик does not necessarily sound especially cute or childish. It can simply be the normal conversational choice.

So:

• Возьми зонт = also correct
• Возьми зонтик = very natural, everyday, slightly softer in tone

This sentence is addressed to one person.

You can tell from the verb forms:

• возьми = singular command
• промокнешь = you singular will get soaked

To say it to more than one person, you would use plural forms:

• Возьмите зонтик, а то промокнете.

That means Take an umbrella, or else you’ll get soaked when speaking to several people, or politely to one person.

Yes, absolutely.

• Если не возьмёшь зонтик, промокнешь.
• If you don’t take an umbrella, you’ll get soaked.

This version is more explicit because it uses a full if clause.

Compared with that:

• Возьми зонтик, а то промокнешь. = more direct, natural warning in conversation
• Если не возьмёшь зонтик, промокнешь. = more logically spelled out

Both are correct, but а то is very common in everyday speech.

It sounds like a normal, practical warning or piece of advice.

Depending on intonation, it can feel like:

• caring: Take an umbrella, otherwise you’ll get soaked
• slightly insistent: Seriously, take one
• mildly parental or familiar

It is not unusually harsh. The tone is mainly determined by context and voice, not just the words themselves.