how to use the word by in the following sentence

Question

I was taught that both present participle(doing something) and prepositional phrase (by doing something) can act as adverbial modifies the verb in the main clause, and can indicate in what(which) way or method something was done or achieve. Does it mean that they are interchangeable in this kind of situation?

For example:

1) You can avoid a lot of problems giving in to her, and you can also cause a lot of problems giving in to her.

2) You can avoid a lot of problems by giving in to her, and you can also cause a lot of problems by giving in to her.

1. Is there any differences between 1) and 2)？

2. How will you understand the first sentence when you see it?

3. How to decided when we need the word by to make present participle ?

My understanding is if the meaning still clearly expressed after the word by was dropped. Then we can left it out, but if the meaning of the sentence will be ambiguous, and then we have to keep the word by.

As for the sentence:

1. He celebrates the Chinese New Year by playing fire works.

Can I drop the word by?

Answer 1

There is no difference in the example you have provided, but the second sentence (using by) is much clearer.

In some cases, it may cause ambiguity if you do not use by, for example:

You can avoid a lot of problems travelling on a train

This sounds like there are a lot of a problems when you travel on a train, and it is possible to avoid them.

You can avoid a lot of problems by travelling on a train

This sounds like there are a lot of problems which you can avoid if you travel on a train.

So there is some difference.

Also your last example, we don't say playing fireworks. We usually say watching fireworks, or launching fireworks depending on if we are just watching or actually participating. Also fireworks is one word.

By is optional in that case:

He celebrates the Chinese New Year by watching fireworks.

He celebrates the Chinese New Year watching fireworks.

Although they have the same meaning there is always a little more emphasis when we use by. In fact, in the case of the fireworks, using by makes it sound like a ritual or habit, whereas without by, it sounds like he is less concerned with how he celebrates.