You must start by a plan/by planning.

[atabitaraf]

You must start by .................. (reference: Mindset3, page 152)
A. a plan
B. planning

I couldn't explain why A is not correct.

 

[Barque]

You need an action after "by", and so "planning" fits.

You could however say "You must start with a plan".

 

[teechar]

You need an action after "by",

That need not always be the case. For example,
The man was hit by a passing motorist as he crossed the road.
The event was marked by intense debate and discussion.
If you are covered by a plan, then your out-of-pocket expenses will be minimal.

I couldn't explain why A is not correct.

That pattern (start+by+a) (where start is a verb) is unnatural. The verb start can be followed by:
1- a full infinitive:
The trees will start to bear fruit about four years later.

2- a gerund:
You must start by planning your itinerary for the holiday.

 

[Barque]

That need not always be the case.

I was talking about that specific sentence alone, to explain why "planning" is better than "a plan". I wasn't putting forward a general theory or rule.

 

[atabitaraf]

You must start by .................. (reference: Mindset3, page 152)
A. a plan
B. planning

I couldn't explain why A is not correct.

I reckon the structure of 'by+a noun' is not of any English type, unless the statement is a passive one: by + an agent.

It is also correct to use 'by + process' like: by post, by email, etc.

The structure of 'by+vehicle' is another well-known usage as well.

Please help to extend them if possible.

 

[teechar]

I reckon the structure of 'by+a noun' is not of used in any variety of English type, unless the statement is a passive one: by + an agent.

No. That is not always the case. For example:
She focused on a complaint by a consumer group and ignored everyone else.
They won by a margin never seen before.
The central bank will likely cut rates by a percentage point at their next meeting.

It is also correct to use 'by + process' like: by post, by email, etc.

Yes.

The structure of 'by+vehicle' is another well-known usage as well.

Yes.

Please help to extend them if possible.

This was the highest jump by a woman.
etc.

 

[atabitaraf]

No. That is not always the case. For example:
She focused on a complaint by a consumer group and ignored everyone else.

Thanks Teechar, only on this statement, I would call it a passive use: 'made by a consumer'.