Gerunds (-ing form used as a noun) and infinitives (to + base verb) are verbals — verb forms that function as other parts of speech. A gerund acts as a noun: "Swimming builds endurance" (subject), "She enjoys swimming" (object). An infinitive can function as a noun (To swim is refreshing), an adjective (a desire to swim), or an adverb (She trained to swim faster). Some verbs take only gerunds (enjoy, avoid, consider), some take only infinitives (want, decide, hope), and some take either with a change in meaning — "I stopped smoking" (quit the habit) versus "I stopped to smoke" (paused in order to smoke).
Sort common verbs into three categories — gerund-only, infinitive-only, and either — and memorize the high-frequency patterns. Then practice with verbs that change meaning depending on which form follows, writing sentence pairs that highlight the difference.
You already know that verb phrases contain a main verb and its complements, and that nouns are words that name people, places, things, or ideas and that can fill subject or object slots in a sentence. Gerunds and infinitives are the intersection of both: they are verb forms repurposed to do noun work (and occasionally adjective or adverb work). The technical name for this category is verbals — verb-derived forms that function as other parts of speech.
A gerund is simply the -ing form of a verb used as a noun. "Swimming is excellent exercise" — "swimming" is the subject of the sentence, occupying the slot where a noun like "exercise" or "basketball" could equally sit. You can test this: replace "swimming" with a noun and the sentence still works. "She enjoys swimming" — "swimming" is a direct object. The key insight is that a gerund inherits some verb properties (it can take objects: "She enjoys swimming laps") while also filling noun slots. Contrast this with the present participle, which is morphologically identical (-ing) but functions differently — in "She was swimming," the -ing form is part of a progressive verb; in "the swimming pool," it's an adjective. Form alone doesn't determine function; position and role in the sentence do.
An infinitive is the to + base verb form. It's more flexible than a gerund: it can function as a noun ("To err is human"), an adjective ("She had no reason to worry"), or an adverb ("He trained hard to qualify"). The adverbial use is particularly useful — "She ran to catch the bus" — where the infinitive explains the purpose of the running. Infinitives can be thought of as potential or prospective actions, which is why they appear after verbs of desire, intention, and decision (want, plan, decide, hope, need).
The practical challenge is that many verbs are selectionally specific: they require either a gerund or an infinitive as their complement, not both. "I enjoy swimming" is correct; "I enjoy to swim" is not. "I want to swim" is correct; "I want swimming" is ungrammatical. The important — and genuinely interesting — subclass is verbs that accept both forms but with a meaning shift. "I stopped smoking" means the habit ended; "I stopped to smoke" means I paused my current activity in order to smoke. "She remembered locking the door" means she has a memory of doing it; "She remembered to lock the door" means she didn't forget to do it. The gerund points backward (a completed or habitual action), the infinitive points forward (an intended or future action). Once you internalize that distinction, the pattern of which verbs take which form becomes less arbitrary and more principled.