For example:
* running, swimming, eating, selling, hoping
are all gerunds.
But
* run, ran, to run, swim, swam, swum, eaten, sold, hoped
are not gerunds.
It is important to understand that gerunds function as nouns, but are derived from verbs. In a sense, they are part noun and part verb, but grammatically, they are considered nouns. For example, gerunds can be subjects or objects of a sentence, but they cannot function by themselves as verbs. For example:
* Running is great exercise. (gerund as subject)
* I don't like running. (gerund as object)
But they cannot be used alone as verbs:
* Tomomi *running two miles every morning. (This sentence needs a verb, such as runs.)
Gerunds may cause a bit of confusion because they look exactly like present participles; for example, if you see the word running all by itself, there's no way you could know whether it's a gerund or a present participle. You'd need to look at the sentence to figure it out. Look at these simple examples:
* Running is tiring. (running is a gerund.)
* I used to love running. (running is a gerund.)
* Running quickly, Tomomi won the race. (running is a present participle..)
* The running students passed a sleeping rabbit. (running is a present participle..)