When/why is "ne" used by itself in dependent clauses?

Question

What are the guidelines for when to use "ne" in phrases like the following:

Il est bien moins spectaculaire qu'on ne le pense généralement.

The "ne" doesn't seem to correspond to anything in English, so it's not clear to me when I should be adding it.

Answer 1

Right after posting the question, I discovered that this is the ne explétif (Expletive ne), which is "used without being needed for the meaning or syntax of a sentence." according to Le Petit Robert de la langue française.

So the ne explétif does not add any meaning – negative or otherwise – to the sentence; it’s just there to draw attention to what precedes it. It’s formal and optional, and used after certain verbs and expressions that have a negative meaning, in either sense of the word: negative as in bad (fear, warning) or negative as in negated (denial, doubt).

Some examples:

• Évitez qu’il ne vous voie. Avoid letting him see you.
• Avant que vous ne preniez une décision … Before you make a decision …
• J’ai moins de travail que vous n’en avez. I have less work than you (do).
• Rarement n’ai-je vu un tel spectacle. I’ve rarely seen such a spectacle.

The most important thing to remember is that the ne explétif does not negate the verb it precedes; for that, you need pas. I found this useful comparison:

• J’ai peur qu’il ne le fasse. I’m afraid he’ll do it.
• J’ai peur qu’il ne le fasse pas. I’m afraid he won’t do it.

The ne explétif can be used in the following scenarios:

1) Verbs and expressions of doubt, fear, and prevention (all or which require the subjunctive)

• appréhender que – to dread, fear that
• avoir peur que – to be afraid that
• contester que* – to dispute that
• craindre que – to fear that
• douter que* – to doubt that
• empêcher que – to prevent that
• éviter que – to avoid that
• s’en falloir que – to almost (do/happen)
• il dépend de (stressed pronoun or indefinite pronoun) que* – it depends on (him, you, this, etc.) whether
• il y a un danger que – there’s a danger that
• il y a un risque que – there’s a risk that
• nier que* – to deny that
• prendre garde que – to be careful, to watch out that
• redouter que – to dread that
• risquer que – to risk, be at risk that
• tenir à ce que* – to be anxious that, to insist that
• trembler que – to be terrified that

*These call for the ne explétif only in negative or interrogative constructions.

2) Conjunctive phrases

• à moins que – unless
• avant que – before
• de crainte que – for fear that
• de peur que – for fear that
• plutôt que – rather than, sooner than
• sans que** – without (Only when the verb before sans que is negated)

These phrases all require the subjunctive as well.

**Only when the verb before sans que is negated.

3) Comparatives of inequality

• autre que – other than
• autrement que – differently than
• meilleur que – better than
• mieux que – best than
• moindre que – least than
• moins que – less than
• pire que – worse than
• plus que – more than

These might require the subjunctive and may be used with a neuter object pronoun (le).

4) Rarely, when it’s at the beginning of a sentence.

Source: https://www.lawlessfrench.com/grammar/ne-expletif/

Answer 2

this french sentence here, if you translate it, means "He is less spectacular than we think generally". It's mean you are contradictory with your thought and the "ne" here is for proving it. I hope i helped you, and sorry if you don't understand something, i'm a french guy trying to improve his english :)

Answer 3

In general ne is used in four cases without negative force:

1. With à moins que, de peur (crainte) que;
2. In que clause in comparison;
3. After verb of fearing e.g. craindre;
4. In the structure ne...que.

Each one is briefly discussed below.

1) Where à moins que or de peur (crainte) que introduce a possibility in the affirmative, ne (without negative force; ne exclamatif) is present before the verb. E.g.

Nous sortirons à moins qu'il ne pleuve [pleuvoir->il pleut->qu'il pleuve [subjunctive). See the reply of @Bill Hayden.] We will go out unless it rains.

Il s'est caché de peur que son père ne l'a battu. He hid for fear that his father might beat him.

2) Ne (stil without negative force) is present in the que clause in comparative sentences as follow:

Il parle plus qu'il ne travaille. He talks more than he works.

Il court plus vite que vous ne pensez. He runs faster than you think.

Une chose plus commune que l'on ne croit. A thing more common than one thinks.

But no ne is present in the que clause in sentences of this kind when the main clause is negative.

3) After verb of fearing preceding the subjunctive as follows:

Je crains qu'il n'aille trop vite. I fear he may go fast.

Je crains qu'il ne vienne. I fear he will (may) come.

Sa mère craignait qu'il n'y soit mécontent. His (or her) mother was afraid that he might be unhappy there.

J'ai peur qu'il ne revienne pas. I am afraid he will not come back.

4) Ne...que = seulement (but, only)

Je ne vous demande que votre amitié. Je vous demande votre amitié seulement. I ask but (only) for your friendship.

Il ne sort que le dimanche. He goes out only on Sundays.

Tu ne me fais la cour que pour t'amuser. You only pay attention to me to amuse yourself.

References:

1) H. Ferrar: A French Reference Grammar, 1967 (Oxford University Press).

2) M. Offord: A Student Grammar of French, 2006. (Cambridge University Press).