1. I felt it my duty to help him.

2. I flet it my duty helping him.

3. I make it a rule to get up at six.
4. I make it a rule getting up at six.

5. I found it easy to memorize the poems.
6. I found it easy memorizing the poems.

7. We consider it foolish of him to try it again.
8. We consider it foolish his trying it again.
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Can we use the gerund instead of the to-infinitive? Are they all grammatical? It seems that the to-infinitive is used more frequently.

while syntactically gerunds are equivalent to infinitives in these uses, in practice, I doubt you will find anyone using the constructions in 2 and 4. I have heard #6. I guess because the word preceding the gerund is an adjective rather than a noun.

#7 and 8 are both common, but more often I have heard 7.

Didn't I already answer this earlier today?

Yes, in the given sentences, we can use both the gerund and the to-infinitive forms after the phrase "it + adjective." However, the to-infinitive form is generally more common.

Let's go through each sentence:

1. I felt it my duty to help him.
Here, the phrase "it my duty" is followed by the to-infinitive "to help." This is the more common construction.

2. I felt it my duty helping him.
Here, the gerund form "helping" is used instead of the to-infinitive. While grammatically correct, this sentence may sound somewhat less natural to native English speakers.

3. I make it a rule to get up at six.
In this sentence, "it a rule" is followed by the to-infinitive "to get up." This is the preferred construction.

4. I make it a rule getting up at six.
This sentence uses the gerund form "getting up" instead of the to-infinitive. This construction may sound less common.

5. I found it easy to memorize the poems.
The phrase "it easy" is followed by the to-infinitive "to memorize." Again, this is the more frequently used form.

6. I found it easy memorizing the poems.
In this case, the gerund form "memorizing" is used instead. Though grammatically correct, this sentence may not sound as natural.

7. We consider it foolish of him to try it again.
"it foolish" is followed by the to-infinitive "to try." This is the more common construction.

8. We consider it foolish his trying it again.
Here, instead of the to-infinitive, the possessive pronoun "his" is used with the gerund form "trying." This structure is less common and may sound slightly awkward to native English speakers.

So, while both the gerund and to-infinitive forms can be used in these sentences, the to-infinitive is generally preferred and more commonly used in English.