Употребите герундий в функции подлежащего.

1. It's so uncomfortable to sleep on the floor.

2. It's great to meet old friends.

3. It's impolite to speak in a loud voice.

4. It's always useful to think.

Я думаю, что тут скорее всего не герундий, а инфинитив надо в функции подлежащего, но приведу 2 примера с герундием и инфинитивом.

Герундий

1. Sleeping on the floor is so uncomfortable.

2. Meeting old friends is great.

3. Speaking in a loud voice is impolite.

4. Thinking is always useful.

Инфинитив

1. To sleep on the floor is so uncomfortable.

2. To meet old friends is great.

3. To speak in a loud voice is impolite.

4. To think is always useful.