The Basic Pattern

After most adjectives in English, you use the infinitive (to + verb), not the gerund (-ing form). This is probably good news — it means there's a strong default rule you can rely on.

✅ She was happy to help with the project. ❌ She was happy helping with the project.

✅ This book is difficult to read without a dictionary. ❌ This book is difficult reading without a dictionary.

The structure looks like this: subject + be + adjective + to-infinitive. It works with a huge range of common adjectives: glad, ready, willing, afraid, surprised, easy, hard, impossible, important, necessary, likely, certain, pleased, shocked, relieved, eager, reluctant… the list is long.

Think of it this way: the adjective describes a feeling or quality, and the infinitive explains what about or why.

I'm afraid to ask her. (The infinitive tells us what I'm afraid of doing.) It's impossible to finish this today. (The infinitive tells us what's impossible.)

When the Gerund Appears Instead

Here's where it gets interesting. A small group of adjectives take a preposition after them, and whenever a preposition is followed by a verb, that verb must be a gerund. So the pattern is: adjective + preposition + gerund.

✅ He's good at solving puzzles. ❌ He's good at to solve puzzles.

✅ I'm tired of waiting for the bus. ❌ I'm tired of to wait for the bus.

The gerund isn't really "chosen by the adjective" — it's forced by the preposition. Common combinations include: interested in, fond of, used to, capable of, responsible for, worried about, keen on, bored with. If you spot a preposition between the adjective and the verb, use the -ing form. Every time.

The "Used to" Trap

This is the single most confusing point at this level. Compare:

✅ I used to walk to school. (= I walked to school regularly in the past — used to + infinitive, a past-habit structure) ✅ I'm used to walking long distances. (= I'm accustomed to walking — be used to + gerund)

These look almost identical on the page, but they're completely different structures. In the second sentence, used is an adjective meaning "accustomed," to is a preposition (not part of an infinitive), and the verb after it must be a gerund. Test it by substituting a noun: "I'm used to the noise" works, so to is a preposition here.

The same logic applies to "look forward to" and "be accustomed to" — the to is a preposition, so a gerund follows:

✅ We're looking forward to meeting you. ❌ We're looking forward to meet you.

The Mistake That Costs Marks

The most common error B1–B2 learners make is applying the default infinitive rule even when a preposition is present. They write things like:

❌ She is interested in to learn French. ✅ She is interested in learning French.

❌ I'm not capable of to do this alone. ✅ I'm not capable of doing this alone.

The fix is mechanical: find the preposition first. If the adjective is followed by of, about, at, in, on, for, with, or to acting as a preposition, use the gerund. If there's no preposition and the infinitive comes straight after the adjective (happy to see, easy to understand), use the infinitive.

One quick diagnostic: try inserting a noun or pronoun after the word to. If the sentence still makes grammatical sense ("I'm used to it," "She's committed to the plan"), then to is a preposition and you need the gerund. If it doesn't make sense ("I'm happy to it"?), then to is part of the infinitive.

Keep this test in your back pocket — it resolves nearly every tricky case you'll meet at this level.