'For' is used to express a duration of time. 'Since' is used to identify the specific starting point of a period that continues until now or another reference point. The difference is between measuring an amount of time and naming when that time began. English relies on this contrast when talking about ongoing situations.

Rule
Use 'for' to express a length of time.
Use 'since' to express when a period started.

'For' introduces a duration, which is a measured time span. The duration can be short or long, and it does not require a starting date. Typical complements include hours, days, months, years, minutes, a long time, and a while. 'For' answers the question 'How long?' without specifying when the period began.

Word/Phrase	Definition
for	Use to show how long something lasts.
two hours	A duration of two hours.
a week	A duration of one week.
a long time	An unspecified but extended duration.

'Since' introduces a starting point in time. The complement of 'since' is usually a date, a time, an event, or a specific moment. 'Since' is most common with present perfect and present perfect continuous when the situation continues to the present. 'Since' answers the question 'Since when?' by naming the beginning.

Word/Phrase	Definition
since	Use to show when a period began.
Monday	The day that marks the start of the period.
2020	A year that functions as a starting point.
this morning	The current day's morning as a starting point.

English often uses present perfect or present perfect continuous with 'since' to describe situations that started in the past and continue now. With 'for,' present perfect and present perfect continuous are also common when the result or state is still true. The key is that 'since' anchors to a starting point, while 'for' measures the length.

Rule
With 'since,' use a tense that connects past and present.
With 'for,' use a tense that emphasizes duration, often present perfect.

Certain types of phrases commonly follow 'for' and 'since.' 'For' is typically followed by expressions of quantity or duration words. 'Since' is typically followed by calendar dates, days of the week, clock times, and named events. Choosing the right complement helps maintain the intended meaning.

Word/Phrase	Definition
for three days	A duration of three days.
since last year	The period starting from last year.
for a while	A moderately short, unspecified duration.
since Monday morning	The period starting from Monday morning.

In connected speech, 'for' is often reduced to a weak form, especially in rapid or casual English. 'Since' typically keeps its full vowel and clear final sounds, including the 's' and 'n' cluster. Clear pronunciation helps distinguish the two words, especially in fast speech.

Word	Notation	Description
for	/fə/	The vowel is reduced to a schwa in fluent speech.
since	/sɪns/	The vowels and final consonants are fully pronounced.