הוא עובד הרבה, אבל היום הוא בבית.

A natural pronunciation guide is:

Hu oved harbe, aval hayom hu babayit.

Word by word:

• הוא = hu
• עובד = oved
• הרבה = harbe
• אבל = aval
• היום = hayom
• בבית = babayit in this sentence

In present-tense Hebrew, sentences like he is at home usually do not use a separate word for is.

So:

• הוא בבית = literally he at home
• natural English: he is at home

This is very normal in Hebrew.
If you wanted past or future, Hebrew would use forms of to be:

• הוא היה בבית = he was at home
• הוא יהיה בבית = he will be at home

But in the present, the is/am/are is usually left out.

עובד is the masculine singular present-tense form of the verb לעבוד = to work.

Because the subject is הוא = he, the masculine singular form is used.

Related forms:

• הוא עובד = he works / he is working
• היא עובדת = she works / she is working
• הם עובדים = they work / they are working
• הן עובדות = they work / they are working for a feminine group

So עובד agrees with the subject in gender and number.

It can mean either one.

Hebrew present tense does not usually make the same clear distinction that English does between:

• he works
• he is working

So הוא עובד הרבה can mean:

• he works a lot (habitual)
• he is working a lot (right now / these days)

The context tells you which meaning is intended.

In this sentence, because of אבל היום = but today, the first clause is most naturally understood as a general fact: he works a lot.

Here הרבה means a lot.

In הוא עובד הרבה, it functions like an adverb:

• עובד הרבה = works a lot

This is very common in Hebrew.
You can think of הרבה as meaning a lot / much / many, depending on the context.

Examples:

• אני לומד הרבה = I study a lot
• יש הרבה אנשים = there are many people

So in your sentence, it modifies the verb עובד.

Yes, historically it is built from יום = day with ה־, but in modern Hebrew היום is very commonly used as the ordinary word for today.

So:

• היום = today

In other contexts, the same written form can mean the day, but in this sentence it clearly means today because it is being used as a time expression:

• אבל היום הוא בבית = but today he is at home

Because Hebrew normally states the subject again in a new clause, especially after a word like אבל = but.

So:

• הוא עובד הרבה
• אבל היום הוא בבית

This feels natural in Hebrew, just as English repeats he:

• He works a lot, but today he is at home.

Also, in הוא בבית, there is no present-tense verb like is, so keeping הוא makes the clause clear and complete.

Hebrew word order is fairly flexible, and time expressions often come early in the clause.

So אבל היום הוא בבית is a very natural way to say:

• But today, he is at home

Putting היום early highlights the contrast:

• usually he works a lot
• today is different

So the order helps emphasize the time contrast.

בבית is made from:

• ב־ = in / at
• בית = house / home

In this sentence it means at home or in the house.

In normal unpointed Hebrew writing, these parts are written together as one word: בבית.

In this sentence, it is understood as בַּבַּיִת (babayit), which includes the idea of the house/home. That is why it means something like:

• in the house
• at home

So הוא בבית is the standard Hebrew way to say he is at home.

Yes. אבל is the most common everyday word for but in Hebrew.

So:

• אבל = but

In your sentence it connects two contrasting ideas:

• הוא עובד הרבה = one situation
• אבל היום הוא בבית = a different situation today

It works very much like English but.