Question

Is there any difference between
these two sentences?
1. How long you have been doing
2. Since when you have been doing this.
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Answer 1

Answer:

For means "from the beginning of the period to the end of the period". since + point: a "point" is a precise moment in time - 9 o'clock, 1st January, Monday. Since means "from a point in the past until now

Explanation:

have been walking since 10 p.m. We have lived here for 20 years. We have lived here since 1985. In the example sentences, both for and since show an event that began in the past and continues into the present.