אם יש הרבה גשם, כדאי להישאר בבית.

אם means if. It introduces a condition:

• אם יש הרבה גשם = if there is a lot of rain

It works much like English if at the beginning of a conditional clause.

יש means there is / there are. So יש הרבה גשם literally means there is a lot of rain.

Hebrew can express this idea in more than one natural way. For example:

• אם יש הרבה גשם = if there is a lot of rain
• אם יורד הרבה גשם = if it is raining a lot / if a lot of rain is falling

So this sentence is using a there is structure rather than an English-style weather subject like it.

In English, weather expressions often need a dummy subject: it is raining, it is cold, and so on.

Hebrew usually does not use a meaningless subject like it in these cases. Instead, it often just uses the verb or another structure:

• יורד גשם = it is raining
  Literally: rain is falling
• יש הרבה גשם = there is a lot of rain

So the sentence sounds completely normal without any word corresponding to English it.

הרבה means a lot of / much / many, depending on context.

In הרבה גשם, it means a lot of rain or much rain.

It is very common in everyday Hebrew and does not change here to match גשם.

Because גשם here is being treated like a mass noun, similar to English rain. You normally say:

• הרבה גשם = a lot of rain

not something like many rains.

So even though English sometimes uses different wording, Hebrew keeps גשם singular in this kind of phrase.

כדאי means something like:

• it is advisable
• it is a good idea
• it is worth it / worthwhile

In this sentence, כדאי להישאר בבית means it’s a good idea to stay at home or one should stay at home.

It is a very common Hebrew way to give advice.

After כדאי, Hebrew normally uses an infinitive:

• כדאי להישאר = it is advisable to stay

So להישאר means to stay / to remain.

This is similar to English patterns like:

• It is good to wait
• It is better to leave
• It is advisable to stay

Yes. If you want to say who the advice is for, you can add a pronoun:

• כדאי לי להישאר בבית = I should stay at home
• כדאי לך להישאר בבית = you should stay at home
• כדאי להם להישאר בבית = they should stay at home

In your sentence, no person is stated, so it has a general meaning: it’s advisable to stay at home.

בבית means in the house / at home.

It is built from the preposition ב־ (in / at) plus בית (house/home).

In Hebrew, בבית is very commonly used where English says at home. So even though the literal idea is connected to house, the natural translation here is at home.

The comma is normal because the sentence begins with a conditional clause:

• אם יש הרבה גשם, ...

Then the main clause follows:

• כדאי להישאר בבית.

This is similar to English writing: If there is a lot of rain, it’s better to stay at home.

Yes. You could also say:

• כדאי להישאר בבית אם יש הרבה גשם.

This means essentially the same thing: It’s advisable to stay at home if there is a lot of rain.

The original version puts the condition first, which is very natural and clear.

Both are natural, but they focus slightly differently:

• אם יש הרבה גשם = if there is a lot of rain
• אם יורד הרבה גשם = if it is raining a lot

The first one talks about the amount of rain more generally.
The second one sounds a little more like an ongoing weather event.

In everyday speech, both are perfectly possible.