When vs. How Long
When and How Long indicate different things. When usually indicates a specific point in time, or something that is considered as a specific point. How long indicates a period or length of time, with a beginning and ending point.
Notice that with when and how long, different tenses and different verbs are often used.
"Actions" usually happen at a point in time, whereas things such as "possession,"status" "condition" "awareness" refer to something that continues over a period of time. (See: Grammar: Action vs. Status.) For example,
A common mistake is using an "action" verb to indicate something that exists over a period of time.
In the above statement, the present perfect tense is used to indicate that you still have the car now. How long can also be used to indicate conditions that existed totally in the past.
Compare the following:
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