Course paper on theoretical grammar

Infinitives and gerunds

Do you like to swim? Do you enjoy learning? If you want to use a verb as a noun, you can turn it into an infinitive or gerund, the noun forms of verbs. An infinitive adds the preposition to in front of a verb’s root form. Although technically two words, an infinitive acts as a single word, usually a noun, but sometimes also an adjective or adverb. 
To forgive is divine. (noun)
Bring a snack to eat if you get hungry. (adjective describing “snack”)
Alternatively, you can turn the verb into a gerund by adding -ing, identical to the present participle. A gerund is strictly used as a noun, and occasionally you can use them to create gerund phrases, which act as a single unit to modify the gerund.
Running is too difficult. 
Studying the cosmos is not the same as understanding the cosmos. 
So what’s the difference between infinitives and gerunds? Often, they are interchangeable—both infinitives and gerunds can act as subjects and direct objects. 
Living with a dog changes your outlook on life. (correct)
To live with a dog changes your outlook on life. (also correct)
However, only gerunds can be the object of a preposition. 
When I’m not fishing, I’m thinking about fishing. 
Moreover, some transitive verbs only use infinitives as a direct object—for example, want. 
I want to break free. (correct)
I want breaking free. (incorrect)
Although there are others, some common verbs use infinitives. They include the following: choose, decide, hope, plan, prepare, promise, wish.
In general, use the infinitives for situations that are abstract, unreal, or haven’t happened yet. Use gerunds for situations that are specific, real, or have already happened. 
In the mornings, I really like to exercise. (abstract; infinitive)
This morning, I really liked exercising. (specific; gerund)