Осенью мне нравится тёплый джемпер без воротника.

Because Russian often uses the instrumental case without a preposition to talk about a season when something happens.

So:

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

This is a very common time expression pattern. In this sentence, осенью means in autumn / during autumn.

You can think of it as a fixed seasonal adverb-like form.

Because нравиться and любить work differently in Russian.

• мне нравится X = I like X
  Literally, this is closer to X is pleasing to me.
• я люблю X = I love X / I like X very much

So in this sentence:

• мне = to me
• нравится = is pleasing
• тёплый джемпер без воротника = the warm collarless sweater

A very literal word-for-word structure would be:

In autumn, to me is pleasing a warm sweater without a collar.

That sounds odd in English, but it is normal in Russian.

Because the verb нравиться takes the person who likes something in the dative case.

So:

• я = I
• мне = to me

Examples:

• Мне нравится книга. = I like the book.
• Мне нравится музыка. = I like the music.
• Мне нравятся книги. = I like books.

With нравиться, the thing liked behaves like the grammatical subject, and the person who experiences the liking goes into the dative.

Because the subject here is джемпер, which is singular.

The verb нравиться agrees with the thing being liked:

• Мне нравится джемпер. = I like the sweater.
• Мне нравятся джемперы. = I like sweaters.

In your sentence, the main noun is:

• джемпер = sweater, jumper

Since that noun is singular, the verb is singular too:

• нравится

Because with нравиться, the thing that is liked is the grammatical subject, so it usually appears in the nominative.

Here the subject is:

• тёплый джемпер без воротника

The core noun is джемпер, and its adjective тёплый agrees with it:

• тёплый = masculine singular nominative
• джемпер = masculine singular nominative

So this is not a direct object the way it would be with любить.

Compare:

• Я люблю тёплый джемпер.
  Here джемпер is a direct object, so it would normally be accusative. For an inanimate masculine noun like джемпер, accusative looks the same as nominative.
• Мне нравится тёплый джемпер.
  Here джемпер is the subject, so it is nominative.

Because the preposition без always takes the genitive case.

So:

• воротник = collar
• без воротника = without a collar

This is a standard rule:

• без сахара = without sugar
• без машины = without a car
• без воротника = without a collar

So воротника is simply the genitive singular form of воротник.

It means without a collar.

In clothing descriptions, воротник means a collar or neck part of a garment. So джемпер без воротника is a sweater/jumper that does not have a collar.

Depending on context, English might translate this as:

• a collarless jumper
• a sweater without a collar

It does not necessarily mean something highly technical; it is just a normal descriptive phrase.

Because Russian word order is flexible, and the beginning of the sentence often sets the scene or gives the time frame first.

So:

• Осенью мне нравится тёплый джемпер без воротника.

puts in autumn first, which sounds natural because it establishes the context right away.

You could rearrange the sentence, for example:

• Мне нравится тёплый джемпер без воротника осенью.

This is still understandable, but it sounds a bit less neutral in many contexts. The original order is a very natural way to say it.

Yes. In normal Russian writing, ё is often replaced by е, so you may see:

• тёплый
• теплый

Both represent the same word. However:

• тёплый shows the correct pronunciation more clearly
• теплый is very common in everyday texts

For learners, it is useful to remember that the pronunciation is tyoplyy, not teplyy.

They are close, and in many situations both can be translated as sweater or jumper.

In everyday usage:

• джемпер often means a knitted upper garment, sometimes lighter or simpler
• свитер often suggests a warmer sweater, often with a high neck or heavier knit

But in real life, these distinctions are not always strict. Native speakers may use the terms somewhat loosely depending on style, region, and personal habit.

So for learning purposes, it is enough to understand джемпер here as jumper/sweater.

Because it agrees with джемпер, which is a masculine singular noun.

Russian adjectives must match the noun they describe in:

• gender
• number
• case

So:

• тёплый джемпер = masculine singular nominative
• тёплая куртка = feminine singular nominative
• тёплое пальто = neuter singular nominative
• тёплые вещи = plural nominative

Here джемпер is masculine, so the adjective must also be masculine:

• тёплый

No, not in this sentence.

To say in autumn as a general time expression, the normal form is:

• осенью

You can also say:

• в эту осень in some special contexts, but that usually means something like this autumn in a more specific, marked way
• этой осенью = this autumn, which is extremely common

But for the simple general meaning in autumn, the standard choice is:

• осенью

So the sentence is natural exactly as written.