He must(ride, rides, to ride) a horse.

Rides,потому что с местоимениями He She It добавляется S

Ride, потому что с must глагол всегда употребляется в инфинитиве

1. Она умеет разговаривать на трёх иностранных языках умение что-либо делать)
2. Она умела танцевать, когда ей было 5 лет умение что-либо делать)
3. Ты можешь взять мою книгу. (may - разрешение)
4. Ты должен сказать мне правду. (must - обязанность)
5. Я должен сделать покупки сегодня. Вечеринка будет завтра. (have to - что-то типо обязанности и нужды)
6. Ты можешь не отвечать на мой вопрос, если не хочешь. (have to - обязанность/нужда)
7. Люди должны больше заниматься спортом. (should - совет)
8. Ты не должен ехать на красный свет. (must - обязанность)

1)
1. The translation was finished by the students in time.
2. The fields will be covered with snow in winter.
3. I am always invited to her dinner parties.
4. Her younger brother was often taken for a walk by Betty.
5. The telegram was asked for by the neighbour.
2)
1. This bone will be given to my dog tomorrow.
2. We were invited to a concert last Saturday.
3. Yesterday he was told to prepare a speech.
4. All the texts were looked through yesterday and not a single mistake was found.
3
1. If he worked hard, he would achieve great progress.
2. If he reads 50 pages every day, his vocabulary will increase greatly.
3. You will never finish your work if you waste your time like that.
4. If he doesn't pass his examination, he will not get a scholarship.
5. If he weren't such a book-worm, he wouldn't spend so much time sitting in the library.