अगर मौसम अच्छा नहीं होगा, तो हम घर में रहेंगे।

अगर means if, and तो means then.

Together, they mark a condition-result pattern:

• अगर = introduces the condition
• तो = introduces the result

So the structure is:

• अगर X, तो Y
• If X, then Y

In Hindi, तो is very common in this pattern, but it is sometimes omitted in everyday speech if the meaning is already clear.

This is a very common difference from English.

In English, we usually say:

• If the weather is not good, we will stay home.

Even though the meaning is about the future, English uses the present tense after if.

Hindi often does not follow that pattern. For a real future condition, Hindi commonly uses the future in both clauses:

• अगर मौसम अच्छा नहीं होगा, तो हम घर में रहेंगे।

So this is normal Hindi structure for a future situation.

In Hindi, नहीं usually comes before the verb or verb phrase that it negates.

Here:

• अच्छा होगा = will be good
• अच्छा नहीं होगा = will not be good

So नहीं is negating होगा.

This placement is very common in Hindi:

• वह नहीं आएगा = He will not come
• हम नहीं जाएँगे = We will not go

Because मौसम is a masculine singular noun in Hindi.

The adjective अच्छा agrees with मौसम:

• masculine singular: अच्छा
• feminine singular: अच्छी
• masculine plural: अच्छे

So:

• अच्छा मौसम = good weather

And in the sentence:

• मौसम अच्छा नहीं होगा

Both अच्छा and होगा match the masculine singular noun मौसम.

होगा is the future form of होना (to be) for a masculine singular subject.

Here the subject is मौसम, which is masculine singular, so Hindi uses:

• होगा = will be

Compare:

• मौसम अच्छा होगा = The weather will be good
• स्थिति अच्छी होगी = The situation will be good
  (स्थिति is feminine, so होगी)

So होगा agrees with मौसम.

Because रहेंगे is the future plural form used with हम (we).

• हम ... रहेंगे = we will stay

In standard Hindi, हम normally takes a plural verb.
So:

• मैं रहूँगा / रहूँगी = I will stay
• हम रहेंगे / रहेंगी = we will stay

In your sentence, रहेंगे is the masculine/mixed-gender plural form.
If the speakers were all female, you could also hear:

• हम घर में रहेंगी।

घर में literally means in the house / inside the home.

In this sentence, it suggests staying indoors.

• घर में रहेंगे = we will stay in the house / inside

You could also say घर पर, which usually means at home.

The difference is subtle:

• घर में = in the house, indoors
• घर पर = at home

So घर में रहेंगे is especially natural when the idea is staying inside because of bad weather.

Yes, often it can.

You can say:

• अगर मौसम अच्छा नहीं होगा, हम घर में रहेंगे।

This is still understandable and natural.

Adding तो makes the condition-result relationship more clearly marked, a bit like explicitly saying then in English.

So:

• अगर ..., तो ... = very clear and common
• अगर ..., ... = also possible

Yes, but Hindi is somewhat flexible.

The most common order for this kind of sentence is:

• अगर
  • condition, तो
    • result

So:

• अगर मौसम अच्छा नहीं होगा, तो हम घर में रहेंगे।

This is the most straightforward structure for learners.

Hindi can sometimes move things around for style or emphasis, but this pattern is the safest and most natural one to learn first.

Yes, mostly in style.

• अगर = common, everyday, conversational
• यदि = more formal, literary, or written

So these are both possible:

• अगर मौसम अच्छा नहीं होगा, तो हम घर में रहेंगे।
• यदि मौसम अच्छा नहीं होगा, तो हम घर में रहेंगे।

A learner will hear अगर more often in normal speech.

Usually, in a full neutral sentence like this, नहीं is the normal choice.

• अच्छा नहीं होगा = standard and neutral

न is shorter and can appear in poetry, set expressions, or certain spoken styles, but it is not the default form learners should use here.

So for everyday standard Hindi, नहीं is the right choice.

A very useful pattern is:

• अगर + condition + future, तो + result + future

For example:

• अगर बारिश होगी, तो हम नहीं जाएँगे।
• अगर वह आएगा, तो हम बात करेंगे।
• अगर समय मिलेगा, तो मैं पढ़ूँगा।

So your sentence is a great model for building future conditional sentences in Hindi.