The qualifying infinitive may be used:
1) to qualify a noun like an adjective
- Give me something to drink.
- Give me a chair to sit.
- It was a sight to see.
- This is a thing to admire.
2) to qualify a verb like an adverb
- I came to see you.
- We are going to play the match.
- It is going to rain.
3) to qualify an adjective like an adverb
- The book is nice to read.
- This picture is beautiful to look at.
4) to qualify a sentence
- To tell the truth, you are a fool.
- To be frank, I don’t like him.
When the infinitive is used to qualify a noun, verb or adjective, it is called a qualifying infinitive or gerundial infinitive.
Adjectives / adverbs and the qualifying infinitives are compared below.
- That was a beautiful sight. (Here the adjective beautiful qualifies the noun sight.)
- That was a sight to see. (Here the infinitive to see qualifies the noun sight.)
- He came quickly. (Here the adverb quickly modifies the verb came.)
- He came to see us. (Here the infinitive to see modifies the verb came.)
- This kind of tea is very good. (Here the adverb very modifies the adjective good.)
- This kind of tea is good to drink. (Here the infinitive to drink modifies the adjective good.)