manhhiep2509 wrote:To learn to play a musical instrument and to paint with oil paints both require at least 2 hours of practice a week.

In my opinion, "to learn" is the subject and "to play a musical instrument" and "to paint with oil paints" are preposition phrases that modify the subject.

Please correct my view.
Thanks.

Hi manhhiep2509,

This is a kind of weird sentence. Because you have a plural verb, you must also have a plural subject. The sentence is vague overall. "To play a musical instrument" and "to paint with oil paints" are not prepositional phrases--they are infinitive phrases. I would rephrase the sentence to read something like this:

Learning to play a musical instrument and learning to paint with oil paints (or to do oil painting) both require at least 2 hours a week.

Hope this helps.

As neptune28 has noted, to learn and to play are not prepositional phrases but INFINITIVES.
An infinitive -- TO + VERB -- can serve as a noun.

Infinitive as DIRECT OBJECT:
John learned TO PLAY.
Here, to play serves as the DIRECT OBJECT of learned.
WHAT did John learn?
He learned TO PLAY.

Infinitive as COMPLEMENT:
In the construction X is Y, Y is called the COMPLEMENT.
The complement essentially is the equivalent of the subject.
An infinitive can serve as the complement of a sentence.
The goal is to raise revenue.
Here, to raise revenue is the complement of the goal.
WHAT is the goal?
TO RAISE REVENUE.
This usage is rare.
I would be skeptical of this construction on the GMAT.

Infinitive as SUBJECT:
There are two primary ways -- one rare, one far more common -- that an infinitive will serve as the SUBJECT.

RARE: INFINITIVE + VERB
TO ERR is human.
Here, TO ERR is the subject of is.
Generally, this construction is used to express a GENERAL PRINCIPLE.
I would be skeptical of this construction on the GMAT.

COMMON: IT + TO BE + MODIFIER + INFINITIVE
It is easy TO LIKE MARY.
In this construction, it serves as an EXPLETIVE: a pronoun serving to delay the subject.
Here, the delayed subject is TO LIKE MARY.
The conveyed meaning is as follows:
To like Mary is easy.
But positioning the infinitive-as-subject before the verb results in a construction that is awkward and to be avoided.
In 99% of cases, an infinitive serving as the subject will be preceded by an expletive such as IT:
IT is easy to like Mary.
This is the sort of construction that the GMAT is likely to use when an infinitive serves as the subject of a clause or sentence.