To native French speakers or those otherwise proficient in French- why are these different?

The word "ne" for not, used in the middle ages in both English and French sounds so weak that both languages felt a need to strengthen it. English added "whit", turning "I care ne" into "I care ne a whit" and eventually "I care not".

French added various words according to the sense "Je ne mange mie" (I don't eat a crumb), "Je ne vois point" (I don't see a dot), "Je ne marche pas" (I don't walk a step). Eventually "pas" took over, and while still meaning "step" can now also be used for "not", so Je ne sais (I dunno), Je ne sais pas (I do not know), Sais pas (Dunno). Meanwhile in English "I know not" has been strengthened into "I do no know" or "I don't know".

French has TWO forms -- FORMAL and INFORMAL.

Formal is used when you are speaking in public, or to people you don't know.

INFORMAL is used when you are with your family or friends.

"je ne comprends pas" is FORMAL

The other two are informal. The "ne" is optional.

I would say:

_ '' Je comprends pas '' is familiar

_ '' Je ne comprends pas '' is formal

_ '' Je ne comprends ''. The adverb: Ne, is used alone in literature and in certain proverbs and expressions.

Examples:

Qui ne dit mot consent. ( Anyone who doesn't say a word consents).

Qu'à cela ne tienne ( it's like say: '' Well, let's just say '' )

Also with the verbs: avoir, savoir et pouvoir, followed by that interrogative and an infinitive:

Ex: Il ne sait que dire = Il ne sait pas quoi dire ( = He doesn't know what to say )

With the verbs cesser, pouvoir and savoir:

Ex: Je ne saurais vous dire. = Je ne saurais pas vous dire (like: I wouldn't to say )

I would like to correct the anonymous person's answer and complete it with why it's this way.

• Using "ne...pas" (je ne comprends pas) is the "correct" formulation in Modern Standard French. That's what foreigners are taught when they learn French in school and that's what you will find on formal documents.

• Using just "ne" (je ne comprends) is also technically correct but contrary to what the anonymous person said, this is not an informal formulation. It's actually waaaaaay more formal than "ne...pas" because it's how negations used to be constructed in Old French and it still exists in poetry. But I do not recommend using it at all because you will just sound weird (a little like if you used "thou" and "thee" in English).

• And finally, using only "pas" (je comprends pas) is considered ungrammatical according to the established standard. However, this is also the most common way to form a negation in modern everyday French. It's only TECHNICALLY incorrect but practically, it's de facto a correct formulation.

Now here is the explanation in chronological order for why it's like this.

• Just "ne" : Old French or very formal literary modern French

Initially, old French had negations formed the same way as Italian or Spanish. The word "ne" had full negative value and was sufficient to form a negation.

According to this construction, the contrary of "je comprends" would have been "je ne comprends".

But negations can be modified. For example, "I do not understand" and "I do not understand ever" do not mean the same thing, in the second case, the word "ever" modified the "not" negation.

So here are some modifications you could find about negations to reinforce their meanings.

- ne...jamais = not ever (jamais meant ever)

- ne...rien = not anything (rien meant something)

- ne...plus = not anymore (plus meant more)

- je ne marche (I do not walk) => je ne marche pas (I do not walk a step)

- je ne bois (I do not drink) => je ne bois goutte (I do not drink a drop)

- je ne mange (I do not eat) => je ne mange mie (I do not eat a crumb)

etc

• "Ne...pas" : Standard formal Modern French

Eventually, these modifications began to take more and more importance.

Some of them marked a specific type of negation, for example :

- ne...jamais = never (je ne marche jamais = I never walk)

- ne...rien = nothing (je ne mange rien = I drink nothing)

- ne...plus = not anymore (je ne marche plus = I don't walk anymore)

- ne...que = only (je ne mange que le soir = I only eat on the evening)

And some others just marked negation without being specific, for example :

- je ne bois pas (I do not drink at all)

- je ne vois goutte (I do not see anything)

- je ne comprends point (I do not understand anything)

etc

Eventually, "pas" became the only default modificator of unspecific negations.

So according to this construction, the contrary of "je comprends" is "je ne comprends pas".

This construction is the one you are supposed to use in modern formal French. People stopped using negations with only "ne" hundreds of years ago and it only remains in poetry. That's why it would be extremely strange if someone today told you "je ne comprends".

• Just "pas" : everyday Modern French

Due to being used systematically in negations, words like "jamais", "rien", "plus" or "pas" acquired full negative value. "Jamais" and "rien" are not used anymore to mean "ever" and "something", they only mean "never" and "nothing".

So if you can use words that already convey fully the negative information, not using "ne" does not remove any information. That's why a few hundred years ago, people gradually started to omit the "ne" in informal speech.

Eventually, this became the most common way to form negations in everyday language.

According to this construction, the contrary of "je comprends" is "je comprends pas".

The difference is in the level of formality - the correct way to form a negative is to use ne...pas around the verb. other versions are informal constructions because everybody knows what you mean. There is no separate translation for pas since its function is to form a negative, the rough equivalent of adding not to an English verb. But it is not the only way of forming a negative adverb eg ne...plus means not anymore.