The words for and since are used in sentences where the speaker wants to talk about something that started in the past and continues into the present.

For is used when specifying the amount of time (how long):

• I've had this watch for more than 40 years.
• I've only known her for a few weeks.
• He's been here for 6 months and still can't speak a word of German.
• She's been smoking for a long time. No wonder she coughs so much!

Since is used when specifying the starting point:

• I've had this watch since 1965.
• I've only known her since the beginning of last week.
• He's been here since April and he still can't speak a word of German.
• She's been smoking since she started grade 5. No wonder she coughs so much!

Note: The present perfect or present perfect continuous are needed in such sentences. It is wrong to say:

- I know her for two years.  
- I know her since 2006.  

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