השרוולים של החולצה החדשה רחבים מדי, ולכן אני מעדיפה את הסוודר מכותנה.

Because ה־ is the Hebrew definite article, meaning the.

• שרוולים = sleeves
• השרוולים = the sleeves

In this sentence, the speaker is talking about specific sleeves: the sleeves of the new shirt.

Hebrew usually shows possession with של, which means of or belonging to.

So:

• השרוולים של החולצה החדשה = the sleeves of the new shirt
• literally: the sleeves of the shirt the new

This is a very common way to express possession in modern Hebrew.

Because adjectives in Hebrew must agree with the noun in gender and number.

• חולצה is feminine singular
• so new must also be feminine singular: חדשה

Compare:

• חולצה חדשה = a new shirt
• ספר חדש = a new book

Since the noun is definite here, the adjective is definite too:

• החולצה החדשה = the new shirt

Because it describes השרוולים (the sleeves), which is masculine plural.

• singular masculine: רחב = wide
• singular feminine: רחבה
• plural masculine: רחבים
• plural feminine: רחבות

So:

• השרוולים רחבים = the sleeves are wide

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

So:

• השרוולים רחבים מדי literally looks like the sleeves too-wide
• but it means the sleeves are too wide

This is normal in Hebrew present-tense sentences.

מדי means too in the sense of excessively.

So:

• רחבים מדי = too wide
• יקָר מדי = too expensive
• קשה מדי = too difficult

It usually comes after the adjective or adverb it modifies.

Yes. ולכן is made of:

• ו־ = and
• לכן = therefore / so

So ולכן means and therefore, and so, or simply so, depending on the context.

In this sentence, it connects cause and result:

• the sleeves are too wide
• so / therefore I prefer the cotton sweater

Because the speaker is female.

The verb להעדיף means to prefer. In the present tense, Hebrew verbs agree with the subject’s gender and number.

• אני מעדיף = I prefer (said by a male)
• אני מעדיפה = I prefer (said by a female)

English does not show this difference, but Hebrew does.

In Hebrew, את marks a definite direct object.

Here, הסוודר means the sweater, which is definite, so it takes את:

• אני מעדיפה את הסוודר = I prefer the sweater

Compare:

• אני מעדיפה סוודר = I prefer a sweater
• אני מעדיפה את הסוודר = I prefer the sweater

So את does not mean with here. It is just a grammatical marker.

The prefix מ־ often means from.

So:

• כותנה = cotton
• מכותנה = from cotton / made of cotton

In natural English, we usually translate it as made of cotton.

So:

• הסוודר מכותנה = the sweater is made of cotton

Sometimes Hebrew uses noun combinations, but מכותנה is the clearer and more standard way to say made of cotton in a sentence like this.

• סוודר מכותנה = a sweater made of cotton
• חולצה מכותנה = a cotton shirt

A form like סוודר כותנה may appear in ads, labels, or casual shorthand, but learners should usually use מכותנה.

Hebrew often marks definiteness on each relevant word.

For example:

• החולצה החדשה = literally the-shirt the-new
• both the noun and the adjective get ה־

And in:

• השרוולים של החולצה החדשה

you have definite sleeves, definite shirt, and definite new.

This is normal Hebrew grammar, even though English often uses the only once per noun phrase.

The sentence is structured very naturally for Hebrew:

• השרוולים של החולצה החדשה = subject
• רחבים מדי = description
• ולכן = connector
• אני מעדיפה את הסוודר מכותנה = result clause

So the pattern is roughly:

[subject] + [description] + [therefore] + [new clause]

This is similar to English in meaning, even though some details of the grammar are different.