How to tell which sentences expressing purposes & which ones expressing reasons?

Question

In Collins COBUILD English Usage:

for

If something is for someone, they are intended to have it or benefit from it.

He left a note for her on the table.

She held out the flowers and said, 'They're for you.'

I am doing everything I can for you.

You use for in front of a noun phrase or -ing form when you state the purpose of an object, action, or activity.

Some planes are for internal use, others for international flights.

The mug had been used for mixing flour and water.

You use for in front of a noun phrase when you are saying why someone does something.

We stopped for lunch by the roadside.

I went to the store for a newspaper.

Be Careful!

Don't use 'for' with an -ing form when you saying why someone does something.

Don't say, for example, 'He went to the city for finding work'.

You say 'He went to the city to find work' or 'He went to the city in order to find work'.

People would stroll down the path to admire the garden.

He had to hurry in order to reach the next place on his schedule.

Ok, the dictionary says that "I went to the store for a newspaper." expressing a reason (The reason I went to the store is that I want to buy a newspaper).

However, I would say that "I went to the store for a newspaper." also expresses a purpose (I went to the store with a purpose, that is I want to buy a newspaper)

So, How to tell which sentences expressing purposes & which ones expressing reasons?

For example, I do we know that this sentence "I went to the store for a newspaper." expresses a reason but not a purpose?? a reason & a purpose are almost the same.

Note: we need to know that because the dictionary says:

Don't use 'for' with an -ing form when you saying why someone does something.

Answer 1

The reason the book makes the distinction between "purposes" and "reasons why" is only to prevent a specific grammar error with "for + -ing form". I don't think they did a great job of explaining things, but hopefully this clears things up for you.

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Purposes

You use for in front of a noun phrase or -ing form when you state the purpose of an object, action, or activity.

I think "an object, action, or activity" means noun here, where "for X" is essentially an adjective (see "Purpose Adjectives"). It's not a perfect comparison (there are subtle differences between these pairs), but you should get the idea:

• 1.
  • A bowl for mixing
  • A mixing bowl
• 2.
  • Some planes are for internal use
  • Internal use planes

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Reasons Why

(It's important not to call it "reasons", since the words "reason" and "purpose" are synonyms. The "why" is important.)

Sentences expressing a "reason why" can easily be detected by forming a "why question" and answer pair:

We stopped for lunch by the roadside.

• Why did we stop? For lunch by the roadside.

I went to the store for a newspaper.

• Why did I go to the store? For a newspaper.

You can use this method to see if "for" or "to" is right; "to" is appropriate in this example because the response to 2 doesn't sound right:

He went to the city [to find/for finding] work.

1. Why did he go to the city? To find work.
2. Why did he go to the city? * For finding work.

Note that you cannot say "I went to the store to a newspaper".

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Overlap

Sometimes both "for" or "to" can be used interchangeably without changing the sentence much (if at all):

The mug had been used for mixing flour and water.

The mug had been used to mix flour and water.

Answer 2

• "I went to to the store for a newspaper" (there's no verb after for so the problem of the -ing suffix doesn't exist)

• "I went to the store to buy a newspaper" to buy expresses the reason.

• "I read newspapers for the news" Why do I buy a newspaper? To read the news.

• "I use newspapers for lining a birdcage" Q: What do I use a newspaper for? What is its function? A: To line the bottom of my birdcage.