Plášť a ventilek jsou špinavé.

Because the subject has two things: plášť a ventilek = the tire and the valve. In Czech, just like in English, two nouns joined by a (and) make a plural subject, so the verb must also be plural.

• je = is (singular)
• jsou = are (plural)

So:

• Plášť je špinavý. = The tire is dirty.
• Ventilek je špinavý. = The valve is dirty.
• Plášť a ventilek jsou špinavé. = The tire and the valve are dirty.

This is about adjective agreement.

The adjective špinavý (dirty) has to agree with the whole subject in number and, in the plural, also in gender type.

Both plášť and ventilek are masculine inanimate nouns. When a plural subject is masculine inanimate, the adjective takes the plural ending -é:

• masculine inanimate plural: špinavé

So:

• špinaví is used for masculine animate plural (for example men or animals in some contexts)
• špinavá is used for feminine singular or neuter plural, depending on context
• špinavé is correct here because the combined subject is treated as masculine inanimate plural

Both are masculine inanimate.

• plášť is masculine inanimate
• ventilek is masculine inanimate

That matters because Czech adjectives and past-tense forms often depend on gender.

A useful comparison:

• ten plášť
• ten ventilek

Using ten is a good clue that they are masculine nouns.

They are in the nominative case, because they are the subject of the sentence.

You are answering the question What is dirty? Answer: Plášť a ventilek.

That means they are the doers or main topic of the sentence, so Czech uses the nominative.

Czech normally does not use articles like a, an, or the.

So Czech often says simply:

• Plášť a ventilek jsou špinavé.

and the listener understands from context whether it means:

• A tire and a valve are dirty
• The tire and the valve are dirty

In real usage, context usually makes this clear.

Yes. Ventilek is a diminutive form related to ventil.

A diminutive often makes a word sound:

• smaller
• more specific
• sometimes more natural in everyday speech

In bicycle-related language, ventilek is very common for valve. So even though it is technically a diminutive form, learners should treat it as a very normal everyday word.

No. Plášť can have more than one meaning depending on context.

Common meanings include:

• coat
• outer covering
• tire casing / outer tire in cycling or mechanical contexts

So if this sentence is about a bicycle, plášť most likely means tire rather than coat.

Context is very important with this word.

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

The neutral order here is:

• Plášť a ventilek jsou špinavé.

But other orders are possible for emphasis, for example:

• Špinavé jsou plášť a ventilek.

This might sound more marked or emphatic, something like It’s the tire and valve that are dirty.

For learners, the original order is the safest and most neutral one.

You would make both the verb and the adjective singular.

Examples:

• Plášť je špinavý. = The tire is dirty.
• Ventilek je špinavý. = The valve is dirty.

Notice that with singular masculine inanimate nouns, the adjective is špinavý.

But with both nouns together, it changes to plural:

• Plášť a ventilek jsou špinavé.

A rough English-friendly guide:

• plá sounds a bit like plaa with a long a
• š sounds like sh
• ť is a soft t sound, pronounced with the tongue closer to the roof of the mouth

So plášť is roughly like plaashty without fully pronouncing the final y. The exact Czech ť does not exist naturally in English, so it may take practice.

Also:

• á = long vowel
• š = sh
• ť = soft t

A rough pronunciation guide:

• ventilek: VEN-ti-lek
• špinavé: shpi-na-vay

A few useful details:

• š = sh
• e is usually a clear short e
• é is a long vowel, roughly like ay in careful learner approximations, though not exactly the same as English ay

Stress in Czech is normally on the first syllable:

• VENtilek
• ŠPInavé

In Czech, predicate adjectives commonly come after forms of to be, just as in English:

• je špinavý = is dirty
• jsou špinavé = are dirty

So the structure is:

• subject + jsou
  • adjective

That is completely normal Czech sentence structure.

Yes, it could.

When Czech has a compound subject, plural agreement depends partly on the genders involved. One important rule is:

• if there is a masculine animate noun in the subject, that usually controls the plural agreement
• otherwise, Czech uses the non-masculine-animate plural form

In your sentence, both nouns are masculine inanimate, so the adjective is špinavé.

So this sentence is a good example of the non-masculine-animate plural pattern.