samui yoru wa, sofa de moufu o kakete hon o yomimasu.

Because 寒い is an i-adjective, and i-adjectives can directly modify nouns.

So:

• 寒い = cold
• 夜 = night
• 寒い夜 = a cold night / cold nights

This works a lot like English adjective + noun order, except the adjective comes before the noun in Japanese too.

は is making 寒い夜 the topic of the sentence.

So 寒い夜は has a feeling like:

• As for cold nights, ...
• On cold nights, ...

This is very natural when talking about a habit or what someone usually does in a certain situation.

If you said 寒い夜に, that would mark cold night more simply as a time expression, like on a cold night. That is also possible in some contexts, but 寒い夜は sounds more like a general pattern or habit.

で marks the place where an action happens.

Here, the actions are:

• 毛布をかけて = put on / drape a blanket
• 本を読みます = read a book / read books

Those actions happen on the sofa, so ソファで is used.

A useful contrast:

• ソファで読む = read on the sofa
• ソファに座る = sit on the sofa

So で is for the location of an action, while に is often used for destination, existence, or verbs like sit.

Because 毛布 is the direct object of the verb かける.

In this sentence, かける means something like:

• to put on
• to drape over
• to cover with

So the thing being placed or draped is marked with を:

• 毛布をかける = put on a blanket / drape a blanket

The person receiving the blanket is often omitted if it is obvious. In English, we might say put a blanket over myself, but in Japanese, the myself part is often left unstated.

かけて is the て-form of かける. The て-form is being used to connect two actions:

• 毛布をかけて
• 本を読みます

So the sentence means that the speaker:

1. puts on or drapes a blanket, and
2. reads a book

The て-form often means and, then, or shows actions that are linked together. Here it gives a natural flow: I put a blanket over myself and read.

Yes, the subject is omitted.

Japanese often leaves out the subject when it is clear from context. In a sentence like this, the most natural interpretation is usually:

• I read
• I put a blanket on myself

But depending on context, it could also refer to someone else. Japanese does this much more often than English.

読みます is the polite non-past form of 読む.

Japanese non-past can cover several meanings depending on context:

• present habitual: read
• future: will read

In this sentence, because of 寒い夜は, it most naturally means a habitual action:

• On cold nights, I read...

It does not usually mean am reading right now here. For an action happening right now, Japanese often uses 読んでいます instead.

It can mean either. Japanese nouns usually do not mark singular vs. plural the way English does.

So 本を読みます could mean:

• read a book
• read books

The exact meaning depends on context. In a sentence about a habit, English may translate it either way depending on what sounds natural.

It usually means cold nights in general here.

Because は sets up a topic and 読みます is in the non-past, the sentence sounds like a repeated or typical action:

• On cold nights, I ...
• When nights are cold, I ...

If the speaker wanted to talk about one particular night, the wording would usually be different and the context would make that clear.