Мебельный чехол защищает мебель.

Because мебельный is an adjective, and it has to agree with the noun it describes.

Here it describes чехол, which is:

• masculine
• singular
• nominative

So the adjective also takes the masculine singular nominative form:

• мебельный чехол

If the noun were feminine, neuter, or plural, the adjective would change too:

• мебельная ткань
• мебельное что-нибудь
• мебельные чехлы

Russian adjectives change form much more than English adjectives do.

Not exactly.

• мебель = furniture
• мебельный = furniture-related, for furniture, furniture-type

So мебельный чехол is literally something like a furniture cover or a cover for furniture.

This is very common in Russian: a noun becomes an adjective to describe another noun.

Examples:

• музыка → музыкальный
• школа → школьный
• мебель → мебельный

So мебельный is not a separate basic noun; it is an adjective derived from мебель.

Чехол is in the nominative singular because it is the subject of the sentence — the thing doing the action.

In Мебельный чехол защищает мебель, the cover is the one that protects.

So:

• чехол = subject
• nominative case

This is the dictionary form of the noun:

• чехол = cover, case

Because защищает is the 3rd person singular present-tense form of the verb защищать.

Breakdown:

• защищать = to protect
• защищает = he/she/it protects

Since the subject чехол is singular, the verb is singular too.

Russian present-tense verbs change according to the subject:

• я защищаю = I protect
• ты защищаешь = you protect
• он/она/оно защищает = he/she/it protects
• они защищают = they protect

Here чехол is an it, so защищает is the correct form.

It is the object, and it is in the accusative case, but for this noun the accusative looks exactly like the nominative.

That is very common in Russian.

Here is what is happening:

• мебель is a feminine noun
• it is inanimate
• many inanimate nouns have the same form in nominative and accusative

So:

• nominative: мебель
• accusative: мебель

Even though the form is the same, the function is different.

In this sentence:

• чехол = subject
• мебель = direct object

Because the two words are doing different jobs.

• мебельный describes what kind of cover it is
• мебель is the thing being protected

So the sentence is structurally:

• [furniture-related] cover protects [furniture]

This kind of repetition is normal in Russian and English if the meaning requires it.

Compare English:

• A furniture cover protects furniture.

Even though both come from the same idea, one is an adjective and the other is a noun.

No. Russian has no articles like a, an, or the.

So Мебельный чехол защищает мебель could mean:

• A furniture cover protects furniture
• The furniture cover protects the furniture
• Furniture covers furniture would be less likely because the noun is singular

Which English article is best depends on context.

Russian usually shows definiteness through:

• context
• word order
• intonation
• sometimes extra words like этот = this, тот = that

Yes, Russian word order is more flexible than English word order.

The neutral, straightforward order here is:

• Мебельный чехол защищает мебель.

But other orders are possible, for example:

• Мебель защищает мебельный чехол.
  This would usually mean It is the furniture that the furniture cover protects, with extra focus on мебель.

Because Russian uses case more than word order, the roles are still clear:

• чехол is nominative → subject
• мебель is accusative in function → object

So changing the order often changes emphasis, not the basic meaning.

A helpful stressed version is:

• Мебе́льный чехо́л защища́ет ме́бель.

Approximate pronunciation:

• мебе́льный = mye-BYEL-nyy
• чехо́л = chy-KHOL
• защища́ет = zash-chee-SHA-yet
• ме́бель = MYE-bel

A few pronunciation notes:

• е often sounds like ye at the start of a syllable or after certain consonant conditions
• ч is like ch in chair
• unstressed vowels are often reduced in natural speech

Stress is very important in Russian, because it is not always predictable.

Защищать is the imperfective verb, while защитить is the perfective partner.

• защищать = to protect, to be protecting, to protect in general
• защитить = to protect successfully, to defend, to complete the act of protection

In a general statement like this one, Russian normally uses the imperfective:

• Мебельный чехол защищает мебель.

This sounds like a general truth or regular function:

• A furniture cover protects furniture.

If you used the perfective, it would sound more like a completed result in a specific situation, which would not fit as well here.

Usually, nouns ending in a consonant are masculine.

Since чехол ends in -л, it is normally masculine:

• чехол = masculine

You can also see this from the adjective:

• мебельный has the masculine ending -ый

So the adjective and noun confirm each other.

This is a useful beginner rule:

• consonant ending → usually masculine
• -а / -я → usually feminine
• -о / -е → usually neuter
• soft sign -ь can be masculine or feminine, so that one must often be memorized

Yes, and that is often very natural.

Two common ways to express this idea are:

• мебельный чехол
• чехол для мебели

The difference is mostly stylistic:

• мебельный чехол = more compact, adjective-based
• чехол для мебели = more explicit, literally cover for furniture

Russian often allows both patterns:

• adjective + noun
• noun + для
  • genitive

Both are correct; which one sounds better depends on context and style.