At fault means responsible for a mistake or problem: I was at fault for the delay. Blame can be shared: both drivers were partly at fault. It works for things too: the cable was at fault, meaning the cable caused the problem. It sounds drier and more formal than It's my fault.
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be at fault; be at fault for + noun / -ing; partly / entirely at fault
- I was at fault for sending the invitation to the wrong address.
- The players argued, but both sides were partly at fault.
- The speaker was working correctly; the loose cable was at fault.
What changed
First versionI was at fault for forget to attach the schedule.
RevisionI was at fault for forgetting to attach the schedule.
After at fault for, the plain verb slips in easily, as if for were part of an infinitive. But for is a preposition, and after prepositions English uses -ing: at fault for forgetting. To attach stays as it is, because it follows forget.
It's my fault is a personal, spoken admission. At fault is drier and fits reports, complaints and accident descriptions: the driver at fault is the one who caused the crash. Faulty describes a broken thing: a faulty cable.
Your turn
Admit responsibility for a mistake: I was at fault ___ using the old address.
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for — The reason for the blame follows for: at fault for using. To is not used before using here.
Admit a small mistake at work and say how you'll fix it. Use at fault for with an -ing form.
I was at fault for booking the wrong room, so I'll arrange a replacement.
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