Which conjunction should I use for a consequence, and which should I use for a condition?

Question

Here is an English grammar question from a studying book I bought.

Q. Wash your hands often, _____ you won't catch a cold.

1. or
2. and
3. so
4. if
5. as

It says the answer is only one.
But I think both 'so' and 'and' are suitable in that sentence.
What do you make of this? Which one do you think the answer is?

Here is another question, which more confuses me.

Sentence: I will take you to a nice restaurant if you won't be busy tomorrow.

___________ → ___________

It is said the sentence needs to be corrected, however I have no idead of where the mistake is.
Frankly, I'm afraid this sentence is correct and makes sense.
On the other hand, since this question was from the part of conjunctive particle,
I guess ' if ' might be supposed to be corrected.
But I don't know for sure. So help me, please.

Answer 1

In the first problem, both words are grammatically correct, but "so" is more correct for conveying meaning. Presumably the purpose of the sentence is to describe a cause and effect relationship between washing hands and avoiding illness, but "and" does not indicate cause and effect. The two clauses could be unrelated. You are right to be confused, though, because common sense would lead the listener to assume the cause and effect relationship. Otherwise, why bother mentioning it at all?

The problem with the second example is "if you won't be." The more appropriate phrasing would be

"I will take you to a nice restaurant if you aren't busy tomorrow."

This is a subtle and challenging concept to explain, but I'll do my amateur best!

"I will take you" is a promise made in the present to do something in the future. "You aren't busy" is a condition that will be met in the future, but there is no claim or promise being made in the present.

By contrast, "you won't be busy" is a condition that is met in the present about the future. By using the original wording of "if you won't be busy," the actual message being communicated is something like:

"I will take you to a nice restaurant tomorrow if you agree to not be busy tomorrow."

Answer 2

For the first one

so is best

so is one of the FANBOYS conjunctions (for, and, not, but, or, yet, so)

and could also be correct, but I would not use a comma before it when and means cause and effect:

Touch that horse and I'll shoot you dead.

(If you touch that horse, I'll shoot you dead.)

The second one

I will l take you to a nice restaurant if you are not busy tomorrow.

This sentence states a condition:

If you are not busy tomorrow

then

I will take to a nice restaurant.

You use the present indicative, are not to indicate a likely event.

In this kind of construction, we don't use

*If you will not be busy tomorrow

But obviously we often use will not and tomorrow often together, so that is where the confusion lies.

Answer 3

Check the definition of so sense 8:

8) with the result that; because of this ⇒ "she smiled, so I did too"

Since the second clause would be considered a result of the first one, "so" is the best choice.
and could be used but it simply joins the clauses.

As for:

I will take you to a nice restaurant if you won't be busy tomorrow.

this could be rewritten several ways. It is hard to tell without knowing the lesson topic. But if the topic is related to your first question, I would guess:

I will take you to a nice restaurant since/as you won't be busy tomorrow.