Can anyone please tell me when we use the verb (start) with (by) and when we use it with (with); look at these examples:
Start with/by doing some exercises.

I'll start with/by some push-ups.

Thank you.

osama91when we use the verb (start) with (by)
Use quote marks "start" "by" -- not parentheses (start), (by).

Like this:

... when we use the verb "start" with "by" ...

You need the verb with -ing if you use "by", but if you use "with", you usually connect to a noun.

I'll start by doing some push-ups.
I'll start with some push-ups.

Let's start by doing some exercises.
Let's start with some exercises.

... start with doing ... is all right, but a little strange to my ear.
... start by push-ups ... is wrong.

You can use either one:

Start with doing some exercises.
Start by doing some exercises.
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Thank you, CJ.
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In your example, doing is used as a gerund, which is a noun, not a verb.

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