+0

I'm completely stumped on these 4 sentences. Could someone explain why (1) doesn't work, even though (2) functions perfectly fine despite their grammatical similarity?

And why, linguistically speaking, despite (3) and (4) also sharing the same grammatical structure, they have completely different meanings? (3) being 'I forgot to complete the act of running' vs (4) 'I stopped what I'm doing to run'

(1) I forgot running.
(2) I stopped running.
(3) I forgot to run.
(4) I stopped to run.

+1
Dean WebberCould someone explain why (1) doesn't work,

The English catenative verbs operate very differently from each other. There is no rule, no rhyme nor reason as to why one takes an infinitive and another takes a gerund.

Here is a very good list of them and the patterns:

https://en.wiktionary.org/wiki/Appendix:English_catenative_verbs

Dean Webber(1) I forgot running.

This is grammatical, but pretty much nonsense in meaning, so it is unheard of in everyday usage.

Dean Webber(2) I stopped running.

Perfectly good and normal.

When you are in a race, sometimes you interrupt the run (stop running) to have a drink.

More common are other objects:

My car stopped running. (It broke down,)

My watch stopped running. (The battery died.)

The trains stopped running. (There was a terrible storm and the trains could not operate.)

Dean Webber(3) I forgot to run.

It needs an adverb to complete the idea.

I forgot to run this morning.

Your habit is a morning run three times a week. This morning you forgot to do it.

I forgot to run by the store on the way to work. 
Dean Webber(4) I stopped to run.

I stopped (in order) to run.

The reason why you stopped your car and got out during a road trip was to stretch and get some exercise.

+1
Dean WebberI'm completely stumped on these 4 sentences.

Join the club!

There is no logical explanation for much of language. Languages grow and change over time in such a way that the reasons why some expressions have their current meanings are lost in history, or if not lost, require some heavy lifting to discover.

Dean WebberCould someone explain why (1) doesn't work

It doesn't work well in the past, but it works in a few other contexts with different shades of meaning.

Imperative.

Forget doing the dishes. We're going to a movie!

With 'can'.

After what you've done, young man, you can forget going out this weekend. You're grounded!

With negation.

I'll never forget living in Spain when I was young.

CJ

Teachers: We supply a list of EFL job vacancies
Comments  

Interesting! Thanks for the list of catenative verbs, but I did notice that 'forget' isn't in there.

And I'm still unclear as to the logic behind why 'I forgot running' doesn't work but 'I stopped running' is okay...how would I explain this to a student?

Students: Are you brave enough to let our tutors analyse your pronunciation?

Also, (sorry I couldn't edit my previous reply) why does (3) and (4) have completely different meanings despite exactly the same grammatical structure? If grammar is a means to communicate meaning, then in theory (3) should mean: 'I forgot (what I was doing, in order) to run', but it doesn't mean that....it's bizarre. And as per my question in my other reply, how would you explain this to a student?

Dean Webberbut I did notice that 'forget' isn't in there.

It has the same pattern as "remember" but with the opposite meaning.

Dean WebberAnd I'm still unclear as to the logic

Think about the semantics. How can one put out of your mind "running", an activity of unlimited duration? We can forget someone's name or a word. These are objects or ideas that can go out of one's mind.

It is easy to imagine how a person can quit / stop / cease the activity of running. One minute they are running, the next minute they are standing still.

how would I explain this to a student?

Much depends on their age and on their current level of English. Students need to hear it explained that the language came first, and then rules and analysis came later to try to explain. But sometimes the only explanation is that something is idiomatic.. When I ask them if the same applies to their native language, they agree that it does.

I don;t remember anyone ever disagreeing.

Clive

Students: We have free audio pronunciation exercises.

Thank you that makes sense!