Этот джемпер мягче, чем тот свитер, который я носил вчера.

Yes, they are very close in meaning, and in many contexts both can be translated as sweater.

A common distinction is:

• джемпер = a knitted top, often lighter, sometimes closer to jumper/pullover
• свитер = a sweater, often warmer or heavier, traditionally with a higher neck

In everyday speech, though, many speakers use them quite loosely. In this sentence, the contrast is simply between this jumper and that sweater.

Both этот and тот are demonstratives:

• этот = this
• тот = that

They change form to match the gender, number, and case of the noun.

Here:

• джемпер is masculine singular nominative, so we use этот
• свитер is also masculine singular nominative, so we use тот

So:

• этот джемпер = this jumper
• тот свитер = that sweater

мягче is the comparative form of мягкий.

• мягкий = soft
• мягче = softer

Russian often forms comparatives with special endings instead of using a separate word like more.

Examples:

• быстрый → быстрее = faster
• теплый → теплее = warmer
• мягкий → мягче = softer

So Этот джемпер мягче means This jumper is softer.

чем is used in comparisons and means than in sentences like this.

So:

• мягче, чем тот свитер = softer than that sweater

This is the most straightforward way to make comparisons in Russian:

• Он выше, чем я = He is taller than I am / than me
• Сегодня теплее, чем вчера = Today is warmer than yesterday

After чем, Russian often keeps the noun in the same case you would expect in a normal clause, and here that is nominative:

• чем тот свитер

This is very common and natural.

There is also another comparative pattern in Russian where the thing being compared can appear in the genitive without чем:

• Этот джемпер мягче того свитера

That also means This jumper is softer than that sweater.

So both are possible:

• мягче, чем тот свитер
• мягче того свитера

For learners, the чем + noun pattern is usually easier to recognize at first.

который means which, that, or who in relative clauses.

Here it introduces extra information about тот свитер:

• тот свитер, который я носил вчера
• that sweater, which/that I wore yesterday

So который connects the noun свитер with the clause я носил вчера.

Because it refers to свитер, which is:

• masculine
• singular
• and in this relative clause, который is the direct object of носил? Actually, let's look carefully.

In который я носил вчера, the word который stands for свитер, the thing that was worn. Since носить normally takes a direct object, many learners expect accusative. But for masculine inanimate nouns, the accusative form is the same as the nominative.

So:

• nominative masculine singular: который
• accusative masculine singular inanimate: который

That is why the form stays который.

If the noun were feminine, you would see the difference more clearly:

• рубашка, которую я носил = the shirt that I wore

носил is the past tense of носить, which means to wear.

Why носил?

• It is masculine singular past tense.
• Russian past tense agrees with the subject.
• Since the speaker says я носил, the speaker is male.

If the speaker were female, it would be:

• я носила вчера

As for надевал:

• надевать / надеть = to put on
• носить = to wear

So:

• Я надел свитер = I put on a sweater
• Я носил свитер = I wore a sweater

In this sentence, wore yesterday is the right idea, so носил is correct.

In the present tense, Russian usually does not use a word for am/is/are.

So:

• Этот джемпер мягче literally looks like This jumper softer
• but it means This jumper is softer

This is completely normal in Russian.

Compare:

• Он высокий = He is tall
• Она дома = She is at home
• Книга интересная = The book is interesting

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

The neutral order here is:

• Этот джемпер мягче, чем тот свитер, который я носил вчера.

But other orders are possible for emphasis, for example:

• Тот свитер, который я носил вчера, мягче этого джемпера.
• Вчера я носил тот свитер, который мягче этого джемпера.

However, changing the word order can shift the focus or make the sentence sound less neutral. For a learner, the original version is a very natural standard pattern.

Because Russian normally separates relative clauses with commas.

Here:

• тот свитер, который я носил вчера

The part который я носил вчера is a relative clause describing свитер, so it is set off with a comma.

This is standard Russian punctuation.

Usually no. In English, you can say:

• the sweater that I wore
• the sweater I wore

But in Russian, you normally need a linking word such as который.

So:

• свитер, который я носил = correct
• свитер, я носил = not correct for this meaning

In informal speech, Russians may sometimes use что instead of который in some contexts, but который is the standard and safest choice for learners.

Here вчера most naturally goes with носил:

• который я носил вчера = which I wore yesterday

So it specifies when you wore that sweater.

In other words, the sentence does not mean that the comparison is happening yesterday. It means the sweater being referred to is the one that was worn yesterday.