For, Since, During

For, since, and during all relate to time, but they answer different questions and take different kinds of complements. Mixing them up is one of the most common preposition mistakes at every level.

For — how long

For marks a length of time. It tells you how long something lasts, lasted, or will last.

For works with almost any tense — present, past, future, and all the perfects.

What follows for

Always a duration — a number plus a time word, or a phrase that describes length.

for three hours, for a week, for five months, for two centuries, for ages, for a long time, for ever

Omitting for

In informal speech, for sometimes drops out before a duration when the verb already implies length.

This is natural in conversation but less common in writing. Including for is always safe.

Not with "all"

Do not use for before all day, all night, all week, or all my life. The word all already covers the duration.

Since — from when

Since marks the starting point of something that is still going on (or was still going on at the time you are talking about). It answers the question from when?

What follows since

Always a point in time — a date, a year, a day, a clock time, an event, or a clause describing a moment.

since Monday, since January, since 1999, since 8 a.m., since the wedding, since I graduated

Since and perfect tenses

Since almost always appears with present perfect or present perfect continuous. The logic is straightforward: since marks a starting point in the past, and the action connects to the present — that is exactly what perfect tenses do.

With past perfect, since works the same way but the reference point shifts to the past.

For vs since — the core contrast

Both for and since often appear in the same type of sentence (especially with present perfect), but they take opposite complements.

The same idea, expressed two ways:

Both sentences describe the same situation. For measures the length; since names the start.

A useful test: if you can replace the time phrase with a number (three weeks, ten minutes, a long time), you need for. If you can replace it with a calendar date or an event (Monday, July, last summer, the day we met), you need since.

During — when within a period

During tells you when something happened — specifically, that it happened inside a named period or event. It does not say how long.

What follows during

Always a noun (or noun phrase) that names a period, event, or activity — never a number or a measured duration.

during the meeting, during the summer, during my stay in Rome, during the exam, during the war, during lunch

During vs in

Both during and in can place an event inside a time period, and they often overlap.

When the focus is on something happening at some point within the period, either works. When you want to emphasize the whole period, during is the more natural choice.

During vs for

This is where most mistakes happen. During names the period; for measures the length.

You can combine them in one sentence without any conflict:

The key rule: during is never followed by a number or a measured duration. If you see a number, switch to for.

Common mistakes

For vs since

During vs for

Unnecessary for