Beyond the verb + gerund / verb + infinitive patterns, there are three other situations where the choice between gerund and infinitive is completely predictable. After a preposition, the gerund is always used — with no exceptions. As the subject of a sentence, the gerund is the natural choice. And to express purpose, the infinitive is used. These patterns are highly reliable and cover a large number of everyday sentences.
Before you start — think honestly about your own teaching and experience.
Look at the examples. Answer each question before reading the explanation — this is how your students will learn too.
Read these sentences. All of them contain a preposition followed by a verb. What form does the verb take after each preposition?
After every preposition, the verb always takes the gerund (-ing). This is a completely reliable rule with no exceptions. The prepositions in these sentences are: at, in, without, instead of, after, before. All of them are followed by -ing. 'Good at explaining' (not 'good at to explain' or 'good at explain'). 'Without saying' (not 'without to say'). 'After finishing' (not 'after to finish'). This is one of the most reliable rules in English — preposition + -ing, always. Learning this rule immediately fixes many student errors. Any time a student sees a preposition before a verb, they can be certain the verb must be -ing.
Now read these sentences where the verb is used as the subject. What form does the verb take?
Replacing the gerund with an infinitive as the subject is possible but sounds formal and old-fashioned: 'To swim is good exercise.' In modern everyday English, the gerund is by far the most natural choice as the subject of a sentence. 'Swimming is good exercise' sounds natural. 'To swim is good exercise' is grammatically correct but is the kind of English found in older texts or very formal writing. For teaching purposes: when a verb is the subject of a sentence, use the gerund. This is the natural, modern choice.'
Now read these sentences about purpose. How does the speaker show WHY they did something?
The purpose is expressed using 'to + infinitive' (or 'in order to + infinitive' for emphasis). 'She went to the market to buy vegetables' — the 'to buy' tells us WHY she went. The infinitive of purpose answers the question 'Why?' or 'What for?' 'In order to' has exactly the same meaning as 'to' for purpose — it is simply more formal and emphatic. In speech and informal writing, 'to' alone is preferred. 'In order to' is common in formal writing, academic writing, and when the writer wants to make the purpose very clear. Note: students sometimes use 'for + -ing' for purpose, which sounds like a product specification ('a knife for cutting bread') rather than a personal purpose ('I went to the market to buy bread').'
THE 'LOOK FORWARD TO' TRAP — the most important preposition exception to teach:
'Look forward to' is a very common fixed expression. The 'to' in this phrase is a PREPOSITION — not an infinitive marker. This means the verb after it must be -ing, not the base form.
This error is extremely common, especially in formal writing and emails — which is exactly where 'look forward to' most often appears. Students who have learned that 'to' signals an infinitive automatically write 'to see' — but here 'to' is a preposition and requires -ing.
OTHER 'TO' AS PREPOSITION PATTERNS:
Object to: They objected to changing the timetable.
Look forward to: I look forward to working with you.
Be committed to: She is committed to improving the school.
Be opposed to: He is opposed to using phones in class.
Be used to: They are used to working with limited resources. (accustomed to — NOT past habit)
The test: if 'to' can be replaced by 'towards' or 'in the direction of' in meaning, it is a preposition → use -ing.
Is there a preposition before the verb? → always use -ing (no exceptions). Is the verb the subject of the sentence? → use -ing (more natural than to + inf). Is the sentence explaining WHY someone did something? → use to + infinitive or in order to. Is 'for' before a verb to explain a person's purpose? → wrong — use 'to' instead. Is 'look forward to' in the sentence? → the verb after 'to' must be -ing (to is a preposition here).
Choose the correct form — gerund (-ing) or infinitive (to + base verb) — for each sentence.
Each sentence contains an error. Write the correct version and explain why — then reveal the answer.
Use this sequence directly in class — guided discovery, no textbook needed. Tap each step to mark it done.
STEP 1 — THE PREPOSITION RULE (8 minutes): Write six prepositions on the board: at, in, about, without, before, after. Ask students to complete a sentence for each — using a verb after the preposition. Collect their answers and ask: what form does the verb take after every preposition? Elicit: -ing, always. Write the rule clearly: PREPOSITION + -ING, ALWAYS. Then ask: what about 'look forward to'? Is 'to' a preposition here? Test: 'She is looking forward — towards what? — seeing you.' The 'to' points towards the gerund. So: preposition → -ing.
STEP 2 — ADJECTIVE + PREPOSITION PATTERNS (5 minutes): Teach the most common adjective + preposition patterns as ready-made phrases.
Good at + -ing
Interested in + -ing
Worried about + -ing
Excited about + -ing
Tired of + -ing
Capable of + -ing
Responsible for + -ing
Ask students to produce one sentence with each. This embeds the patterns as whole phrases.
STEP 3 — GERUND AS SUBJECT (5 minutes): Write these sentence starters. Students complete them using a gerund as the subject.
STEP 4 — PURPOSE INFINITIVE (5 minutes): Ask students to describe three things they did this week using the purpose infinitive.
STEP 5 — ERROR HUNT (5 minutes): Write five sentences — some correct, some wrong. Students work in pairs to correct them. Include: before to start → before starting, for buying → to buy, committed to improve → committed to improving, look forward to see → look forward to seeing. Discuss the rule behind each.
Use directly in class — copy, adapt, or read aloud. No printing needed.
For each strategy, choose the option that best describes where you are now.
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