כדאי שנקנה סוודר מכותנה ליום קר, כי סוודר מצמר לפעמים חם מדי במשרד.

כדאי is an impersonal expression that means something like it is כדאי / worthwhile / advisable / a good idea.

In this sentence, כדאי שנקנה... means it would be a good idea for us to buy... or more naturally, we should buy...

A very common pattern is:

כדאי + ש + future verb

For example:

• כדאי שנלך = We should go
• כדאי שתבדוק = You should check
• כדאי שאקנה = I should buy

So here:

• כדאי = it’s advisable
• שנקנה = that we buy / for us to buy

Together: We should buy...

The ש is the word that, but in Hebrew it is often attached directly to the next word.

So:

• שנקנה = ש + נקנה
• literally: that we will buy
• naturally here: that we should buy

After כדאי, Hebrew often uses this structure:

• כדאי שנקנה
• כדאי שתלמד
• כדאי שתחכו

English usually does not say It’s advisable that we buy in everyday speech, but Hebrew does this very naturally.

נקנה here is the 1st person plural future form of the verb לקנות = to buy.

So:

• אני אקנה = I will buy
• אתה תקנה = you will buy
• הוא יקנה = he will buy
• אנחנו נקנה = we will buy

In this sentence, because it comes after כדאי ש..., it has the sense we should buy rather than a plain future we will buy.

A useful note: in unpointed Hebrew, נקנה can also look like another form from a different meaning, but here the context clearly shows it means we will buy / we should buy.

This is a very good question, because the difference is about nuance.

• ליום קר = for a cold day
• ביום קר = on a cold day

In the sentence, ליום קר means the sweater is suitable for use on a cold day. It expresses purpose or intended use.

So:

• סוודר מכותנה ליום קר = a cotton sweater for a cold day

If you said ביום קר, that would sound more like describing when something happens:

• ביום קר אני שותה תה = On a cold day I drink tea

So ליום קר is the better choice here because the meaning is a sweater for cold-day use.

In Hebrew, adjectives usually come after the noun, unlike in English.

So:

• יום קר = a cold day
• סוודר חם = a warm sweater
• משרד גדול = a big office

The adjective also has to agree with the noun in gender and number.

Here:

• יום is masculine singular
• קר is masculine singular

So יום קר is the correct form.

They mean made of cotton and made of wool.

They are built from the preposition מ־ meaning from, attached to the noun:

• מכותנה = from cotton / made of cotton
• מצמר = from wool / made of wool

In English, we often say cotton sweater or wool sweater, but Hebrew very often uses מ־ for material:

• חולצה מכותנה = a cotton shirt
• כובע מצמר = a wool hat
• שולחן מעץ = a wooden table / a table made of wood

So in this sentence:

• סוודר מכותנה = a sweater made of cotton
• סוודר מצמר = a sweater made of wool

Because it is indefinite, not the sweater.

• סוודר = a sweater
• הסוודר = the sweater

The sentence is talking about sweaters in a general or nonspecific way:

• a cotton sweater
• a wool sweater

So Hebrew uses the indefinite form without ה־.

If the speaker meant a specific sweater, they might say:

• הסוודר מכותנה
• הסוודר מצמר

But here the meaning is more general: a cotton sweater would be better...

Hebrew often repeats the noun when making a comparison or contrast, especially when the speaker wants to be clear.

So:

• סוודר מכותנה... כי סוודר מצמר...

This clearly contrasts:

• a cotton sweater
• a wool sweater

English might sometimes avoid repetition, but Hebrew is often perfectly natural with it. The repetition makes the contrast explicit and easy to follow.

You could imagine other ways to phrase it, but the repeated noun is very normal and clear here.

מדי means too in the sense of excessively.

So:

• חם מדי = too warm / too hot
• יקר מדי = too expensive
• גדול מדי = too big

Notice the word order: in Hebrew, מדי usually comes after the adjective.

So:

• חם מדי
• not מדי חם in this basic pattern

This is one of the most common ways to say too + adjective in Hebrew.

Because it describes סוודר, which is a masculine singular noun.

• סוודר = masculine singular
• therefore חם = masculine singular

If the noun were feminine, the adjective would change:

• חולצה חמה = a warm shirt
• שמלה חמה = a warm dress

So in the sentence:

• סוודר מצמר... חם מדי
• literally: a wool sweater... too warm

The adjective agrees with the noun it describes.

In present-tense Hebrew, there is usually no word for is / are in ordinary sentences.

So Hebrew says:

• הוא עייף = he is tired
• החדר קטן = the room is small
• סוודר מצמר חם מדי = a wool sweater is too warm

That is why the sentence does not need a separate word for is.

So this part:

• סוודר מצמר לפעמים חם מדי במשרד

literally looks like:

• a wool sweater sometimes too warm in the office

but naturally means:

• a wool sweater is sometimes too warm in the office

במשרד means in the office.

It is made of:

• ב־ = in
• המשרד = the office

When ב־ comes before ה־, they combine:

• ב + המשרד = במשרד

This kind of contraction is very common in Hebrew:

• בבית = in the house
• בספר = in the book
• במשרד = in the office

So במשרד is simply in the office.

כי means because here. It introduces the reason.

So the structure is:

• כדאי שנקנה סוודר מכותנה...
• כי סוודר מצמר... חם מדי...

In other words:

• We should buy a cotton sweater... because a wool sweater is sometimes too warm in the office.

כי is one of the most common Hebrew words for because.

לפעמים means sometimes, and its position here is very natural in Hebrew.

• סוודר מצמר לפעמים חם מדי במשרד

This means:

• A wool sweater is sometimes too warm in the office.

Hebrew adverbs like לפעמים are often fairly flexible in position, but placing it before the adjective phrase is common and clear.

Compare:

• לפעמים הוא עייף = Sometimes he is tired
• הוא לפעמים עייף = He is sometimes tired

So here the placement is normal and emphasizes that this is not always true, only sometimes.