Осенью мокрая листва прилипает к обуви.

In Russian, seasons can be used in the instrumental case to express time when something happens:

• весной = in spring
• летом = in summer
• осенью = in autumn
• зимой = in winter

So Осенью literally looks like an instrumental-case form, but in practice you learn it as a very common adverbial time expression meaning in autumn / during autumn.

You could also say в осеннее время, but осенью is the normal natural way.

Листва is a collective noun. It means foliage, leaf cover, or the leaves as a mass, not individual leaves counted one by one.

So even though English may think of this as plural, Russian treats листва as:

• feminine
• singular

That is why both the adjective and the verb are singular:

• мокрая листва
• листва прилипает

Compare:

• листва = foliage / leaves as a mass
• листья = leaves as individual items

Because листва is feminine singular, the adjective has to agree with it.

Agreement in Russian means the adjective matches the noun in:

• gender
• number
• case

Here:

• листва = feminine, singular, nominative
• so мокрая must also be feminine, singular, nominative

That is why you get:

• мокрая листва = wet foliage

If the noun were листья instead, then you would say:

• мокрые листья = wet leaves

Because the subject is листва, and листва is grammatically singular.

So:

• листва прилипает = the foliage sticks
• not прилипают, because that would require a plural subject such as листья

Also, прилипает is:

• 3rd person singular
• present tense
• imperfective

In sentences like this, the present tense often expresses a general or repeated situation: this is what happens in autumn.

The verb прилипать / прилипнуть normally takes к + dative, meaning to stick to something.

So:

• прилипать к чему? = to stick to what?

That is why:

• к обуви = to footwear / to shoes

Here обувь changes to the dative case:

• nominative: обувь
• dative: обуви

So the structure is completely normal:

• листва прилипает к обуви = foliage sticks to footwear

Обувь means footwear in a general sense. It is a broad word, like a category.

Depending on context, English might translate it as:

• shoes
• footwear
• boots/shoes more generally

Russian uses обувь as a singular noun, even though in English we often use a plural word like shoes.

If you wanted to be more specific, you might say:

• к ботинкам = to boots
• к туфлям = to shoes
• к сапогам = to boots

But к обуви is a general statement.

This is an important vocabulary distinction.

• листва = foliage, leaves collectively, leafy matter
• листья = leaves, individual leaves

In your sentence, листва sounds natural because the speaker is talking about wet leaf matter on the ground as a mass that sticks to footwear.

Compare:

• На земле лежит мокрая листва. = Wet foliage/leaves cover the ground.
• На дорожке лежат листья. = Leaves are lying on the path.

So листва gives a more mass-like, autumnal feeling.

Russian has no articles like English a and the.

So:

• мокрая листва can mean wet foliage, the wet foliage, or even some wet foliage, depending on context.

Russian expresses definiteness in other ways, such as:

• context
• word order
• stress
• extra words if needed

In this sentence, the meaning is simply understood from context: we are talking generally about wet autumn leaves.

Usually it sounds like a general seasonal observation:

• Осенью мокрая листва прилипает к обуви.
• In autumn, wet foliage sticks to footwear.

The present tense imperfective is often used in Russian for habitual or typical actions, just like English can say:

• In winter it gets dark early.
• Cats like warmth.

So this sentence is not necessarily about one specific moment. It sounds more like this is what typically happens in autumn.

Yes. Russian word order is flexible, although different orders sound slightly different in emphasis.

The original sentence:

• Осенью мокрая листва прилипает к обуви.

is very natural and neutral: first the time, then the subject, then the verb phrase.

Other possible orders include:

• Мокрая листва осенью прилипает к обуви.
• К обуви осенью прилипает мокрая листва.

These are grammatically possible, but the emphasis changes. For a learner, the original order is the best one to copy.

Yes, stress is worth noticing:

• о́сенью
• мо́края
• листва́
• прилипа́ет
• о́буви

A couple of these may be especially useful:

• листва́ has stress on the last syllable
• прилипа́ет has stress on -па́-
• о́бувь / о́буви keeps the stress on the first syllable

Learning the stress together with the word will help your pronunciation a lot.