1. I won first prize in the English speech contest.

2. I won the first prize in the English speech contest.
--------------------------------
What is the difference between the two sentences?

3. I got a nice gift for my birthday.

4. I got a nice gift on my birthday.
-------------------------
In #4, 'on my birthday' is an adverbial phrase. Is that right?
What about in #3? Is 'for my birthday' an adverbial phrase or an adjective phrase? And then what is the use of 'for' in #3? What difference do the two sentences have because of different prepositions?

1 and 2 -- both are fine.

3 means someone gave you a gift for your birthday, but doesn't specify when it was given to you.

4 means someone gave you a nice birthday gift on your actual birthday day!

Both prepositional phrases are adverbial.

1. The difference between sentences 1 and 2 lies in the placement of the definite article "the". Sentence 1, "I won first prize in the English speech contest", implies that you were the winner of the highest prize without specifying the specific ranking of the prize. In contrast, sentence 2, "I won the first prize in the English speech contest", suggests that you won the highest-ranked prize specifically.

2. In sentence #4, "on my birthday" is indeed an adverbial phrase. It describes the time or occasion when you received the gift.

3. In sentence #3, "for my birthday" is an adverbial phrase as well. It provides information about the purpose or reason for receiving the gift.

The use of different prepositions in sentences #3 and #4 results in a subtle difference in meaning. "For my birthday" (sentence #3) implies that the gift was given to celebrate your birthday or as a gift specifically for that occasion. On the other hand, "on my birthday" (sentence #4) simply indicates the time when you received the gift, without specifying any particular relation to the occasion itself.