Этот зонт мокрый, возьми другой.

Because they both agree with зонт.

In Russian, words that describe or point to a noun usually match that noun in:

• gender
• number
• case

Here:

• зонт = masculine, singular
• этот = masculine singular form of this
• мокрый = masculine singular form of wet

So the phrase is:

• этот зонт = this umbrella
• мокрый зонт = a wet umbrella

If the noun changed, the forms would change too:

• эта сумка мокрая = this bag is wet
• это пальто мокрое = this coat is wet

So the endings are there because of agreement.

Russian often leaves out a noun when it is obvious from context.

So возьми другой literally means:

• take another one

But the full idea is:

• возьми другой зонт = take another umbrella

Since зонт was just mentioned, Russian does not need to repeat it.

This works very naturally with adjectives and words like другой:

• Мне не нравится этот. Дай другой. = I don’t like this one. Give me another.

So другой is standing in for другой зонт.

It is accusative in meaning, but for masculine inanimate nouns, the accusative often looks exactly like the nominative.

The verb возьми takes a direct object, so another umbrella is in the accusative case.

The full phrase would be:

• возьми другой зонт

Now look at зонт:

• masculine
• singular
• inanimate

For masculine inanimate nouns, accusative = nominative.

The same thing happens with the adjective:

• nominative: другой
• accusative masculine inanimate: другой

So the form does not change.

Compare:

• возьми другой зонт = masculine inanimate, same form
• возьми другую книгу = feminine, accusative changes to другую
• возьми другого человека = masculine animate, accusative changes to другого

This is about verb aspect, which is very important in Russian.

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

Here, возьми is used because the speaker means a single completed action:

• Take another one.

The idea is not be taking or take in general, but pick up a different umbrella now.

So:

• возьми другой = take another one / pick another one

If you used бери другой, it could sound more like:

• take another one (go ahead, choose this one instead)

That is also possible in some contexts, but возьми is the most straightforward choice for a one-time result.

Because this sentence contains two parts:

• Этот зонт мокрый = This umbrella is wet
• возьми другой = take another one

In Russian, when two clauses are joined without a conjunction like and, a comma is commonly used.

So the comma marks a pause and separates:

1. the statement
2. the command that follows from it

It is very natural Russian punctuation here.

You can think of it as:

• This umbrella is wet, take another one.

In the present tense, Russian usually does not use a verb meaning is.

So instead of saying:

• This umbrella is wet

Russian simply says:

• Этот зонт мокрый

Literally:

• This umbrella wet

This is normal Russian grammar.

The missing is happens in many sentences:

• Он дома = He is at home
• Она врач = She is a doctor
• Книга интересная = The book is interesting

In past and future tenses, forms of to be can appear differently, but in the present tense they are usually omitted.

Yes. Мокрый is the normal everyday word for something wet.

For an umbrella, мокрый is exactly what you would expect:

• мокрый зонт = a wet umbrella

A learner may also meet сырой, but that usually means things like:

• damp
• raw / uncooked
• humid
• sometimes not fully dried

So for an umbrella covered in water from the rain, мокрый is the natural choice.

Yes, Russian word order is flexible, but the original order is the most neutral and natural here.

Original:

• Этот зонт мокрый, возьми другой.

This presents the reason first, then the instruction.

You could also say:

• Возьми другой, этот зонт мокрый.

That puts more emphasis on the command first: Take another one — this umbrella is wet.

Both are understandable, but the original sounds very natural if you first notice the wet umbrella and then tell someone what to do.

The usual stress is:

• Э́тот зонт мо́крый, возьми́ друго́й.

A rough pronunciation guide:

• Э́тот = EH-tuht
• зонт = zont
• мо́крый = MOH-kriy
• возьми́ = vaz-MEE
• друго́й = droo-GOY

A couple of useful points:

• возьми has stress on the last syllable: возьми́
• другой also has final stress: друго́й
• In normal speech, unstressed vowels are reduced, so pronunciation is not always exactly what the spelling suggests

Because nouns in Russian have grammatical gender, and зонт is a masculine noun.

A very common pattern is:

• nouns ending in a hard consonant are often masculine

Since зонт ends in a consonant, it follows that common pattern.

That is why it takes masculine forms:

• этот зонт
• мокрый зонт
• другой зонт

This is grammatical gender, so it is just a property of the noun that other words must match.

Yes. Другой is often used for both.

Depending on context, it can mean:

• another
• a different
• the other

In this sentence, it means:

• another one
• a different one

So возьми другой means:

• take another one
• take a different one

Russian uses другой very broadly, and context tells you which English translation fits best.