Sure, here are the sentences rewritten using "If":

1. **If** they are here, **they are happy**.

Explanation: Combining the two sentences into a conditional form where the condition ("if they are here") leads to the consequence ("they are happy").

2. **If** he wasn't at the seaside, **he didn't swim with his friends**.

Explanation: This sentence suggests a conditional relationship where the condition ("if he wasn't at the seaside") leads to the consequence ("he didn't swim with his friends").

3. **If** we are not busy, **we shall go with him**.

Explanation: This sentence forms a conditional statement where the condition ("if we are not busy") leads to the consequence ("we shall go with him").

4. **If** Nga is at home, **it's raining**.

Explanation: This sentence establishes a conditional relationship where Nga being at home is the condition that leads to the consequence that it's raining.

5. **If** the accident happened, **the two drivers were in a great hurry**.

Explanation: This sentence presents a conditional statement where the occurrence of the accident is the condition that leads to the consequence that the two drivers were in a great hurry.

These rewritten sentences use "If" to express conditional relationships between the events or situations described in each pair of original sentences.