What follows a preposition?
In its report on the first Airbus with shark-skin film, two sentences use the same pattern: The film saves fuel by reducing drag. and Before carrying passengers, the aircraft made a series of test flights. After a preposition such as by or before, the verb takes the -ing form.
Two forms side by side
Learners often mix up two patterns that both describe actions. Compare them:
| preposition + -ing | to + infinitive |
|---|---|
| How something happens, or when (by, before, after, without, instead of) | Why something is done (purpose) |
| The crew saved time by boarding early. | The crew boarded early to save time. |
| Check the flaps before taking off. | Add the film to reduce drag. |
- The preposition stays in front of the verb: instead of waiting, without asking, after landing.
- A subject is not needed after the preposition if it is the same as the main subject: The engineer found the leak by checking the fuel lines.
Watch out for these mistakes
- ✗ Before to carry passengers, the crew tested the aircraft. → ✓ Before carrying passengers, the crew tested the aircraft. (A preposition is never followed by to + verb.)
- ✗ The airline used the old aircraft instead of to wait for a new one. → ✓ …instead of waiting for a new one.
- ✗ The film saves fuel to reduce drag. → ✓ The film saves fuel by reducing drag. (The film reduces drag first; that is how it saves fuel, not why.)
- Be careful with to as a preposition: We look forward to hearing from you. Here to is a preposition, so the verb ends in -ing.
Try these
Choose the form that fits the meaning. Think about how or why before you click.
The film saves fuel ____ drag.
Reducing drag is how the film saves fuel, so you need by + -ing.
Engineers fitted the film ____ fuel.
Here the question is why they fitted the film. To + infinitive gives the purpose.
The aircraft made test flights ____ passengers.
Before is a preposition, so the verb that follows ends in -ing.
The airline brought a spare aircraft ____ a long delay.
The sentence gives the purpose of bringing a spare aircraft, so to + infinitive fits best.
The airline added the film to an aircraft it already owns, ____ for a new one.
Instead of is followed by the -ing form.
The crew finished the checklist without ____ (find) any fault.
Without is a preposition, so the verb after it takes the -ing form: finding.
Without + -ing: the crew finished the checklist and they did not find a fault.
Now make it real
Write one or two true sentences about your own flying or work. Use by, before or after + -ing. Use this frame: I … by …ing / Before …ing, I …
Check your sentences before you look at the samples:
- Is your sentence true, and does it say what you meant?
- Is there a verb with -ing directly after by, before or after?
- Did you avoid to + verb after the preposition?
- Does it start with a capital letter and end with a full stop?
Here are two samples to compare with your own:
- I prepare for night flights by sleeping in the afternoon.
- Before starting the engines, I check that all the doors are closed.
Now read your sentences again. Can you improve one of them, for example by adding a more exact verb?
CEFR Level B1 / ICAO Level 4
Another group of verb patterns is worth studying next: see Structure: ‘Gerunds and infinitives’.
