Differences between "inform, tell, notify, and instruct"

Question

When and how to use these verbs correctly? I believe that even though they sound synonymous, every word has its own proper usage in a sentence.

According to Merriam Webster,

inform- to impart or give information

tell- to make known; to express in words

notify- to give notice of or report the occurrence

Does it mean they are somewhat synonymous and can be used in a sentence interchangeably? For example,

1. I already informed/told/notified him about our plans this September.
2. Please inform/notify our classmates that we will have a long quiz on our History subject.
3. The police notified/told/informed the family that this is not just a mere case of suicide.

Answer 1

They can often be used interchangeably, but tell has a more general and wider meaning and application than inform and notify which means that the latter two can often be replaced by tell but not always vice versa.

Here's an example:

• He told him that changes would be made.
• He notified him that changes would be made.
• He informed him that changes would be made.

They can be used interchangeably in the above sentences, but have a look at this:

• He told his grandson the Christmas story.
• *He notified his grandson the Christmas story.
• *He informed his grandson the Christmas story.

You could say that he informed/notified his grandson of/about the Christmas story, but that would completely change the meaning.

And then there are cases where it is doubtful:

• He told his grandson how he met his wife.
• He notified his grandson how he met his wife.
• He informed his grandson how he met his wife.

I'm sure you'll notice that in this case notify and inform do not really work. They're not appropriate in this more informal context. Here to tell denotes a more story-telling way of conveying information. I doubt the man would tell his grandson in a formal and subjective way how he met his wife, which is what notify and inform mean.

Answer 2

What is being communicated should help to decide which is better, though no doubt there can be crossover. In your examples:

1) I already told him about our plans this September.

because "our plans" would be considered general information.

2) Please inform our classmates that we will have a long quiz on our History subject.

since "we will have a long quiz" is information that they should pay attention to.

3) The police notified the family that this is not just a mere case of suicide.

Official information would generally be considered a notice.

More context would make it easier to decide which is better.