I am confused between the two.
I demand that he leave!
or I demand that he leaves!
I demand that this man leave!
or I demand that this man leaves!
Which is correct with s or without and why?
Technically speaking, only this one is correct:
I demand that he/this man leave!
The reason why that one is correct while the other one is not has to do with the fact that what we're dealing here with is an example of something called a subjunctive mood. Wikipedia defines subjunctive mood as follows:
The subjunctive is a grammatical mood found in many languages. Subjunctive forms of verbs are typically used to express various states of unreality such as wish, emotion, possibility, judgment, opinion, obligation, or action that have not yet occurred.
The English subjunctive mood is a bit messy as it comes in all shapes and sizes: May God rest his soul, God bless America etc. are all instances of the English subjunctive. What we've got in our case here however is an example of a very particular subjunctive called the mandative subjunctive. And this is just one out of many different types of subjunctive mood that exist in English:
The mandative subjunctive is used with clauses that usually, but not always, begin with that and express a demand, requirement, request, recommendation or suggestion. It is indicated by the use of the base form of the verb (e.g. the base form of the verb to love is love) with singular subjects.
It is recommended that the Board approve the policy.
I suggest that she leave the country.
I demand that he pay his share.
- I demand that he leave!
- I demand that he leaves!
These are both examples of what are known as ᴍᴀɴᴅᴀᴛɪᴠᴇ ᴄᴏɴsᴛʀᴜᴄᴛɪᴏɴs. Sentences such as example (1) are known as sᴜʙᴊᴜɴᴄᴛɪᴠᴇ ᴍᴀɴᴅᴀᴛɪᴠᴇs. Examples such as (2) are known as ᴄᴏᴠᴇʀᴛ ᴍᴀɴᴅᴀᴛɪᴠᴇs. There is a third type of mandative called a sʜᴏᴜʟᴅ-ᴍᴀɴᴅᴀᴛɪᴠᴇ.
Mandative constructions use a content clause which tells us about some desired or wanted situation or event (a content clause always has a full verb and a subject). Normally the situation is one that the people in the Subject of the clause want to happen:
In the examples above, the clauses in brackets describe what the judge wanted to happen. In examples like these we call the verb a mandative verb and we call the clauses that occur after them mandative clauses.
Notice that the three examples above use slightly different clauses. The first example, a subjunctive mandative, uses a plain form of the verb. (The plain form of the verb has no tense and no third person 'S'). The second example, a should-mandative, uses the modal verb should. The third example uses the same tenses and inflections that we see in normal clauses. Because there is no special marking of the mandative here, it is known as a covert mandative—where the word covert means hidden.
British English speakers as a whole prefer covert mandatives to subjunctive ones, at least in spoken English. However, American English speakers highly prefer subjunctive mandatives to covert ones, and some American English speakers may even find covert mandatives only marginally acceptable (i.e. they may verge on finding them ungrammatical).
Notes for language junkies
Here is a slide from a study done by the Survey of English Usage at UCL. It compares the frequency of different types of mandative clauses in British English in two corpora. The LLC data is from the 1950s-early 70s. The ICE-GB data is from the early 1990s. In this table, the term ɪɴᴅɪᴄᴀᴛɪᴠᴇ is used to indicate covert mandative clauses.
As you will see, the use of covert ('indicative') mandatives seems to have increased significantly during this period in spoken English. In the modern ICE-BG corpus, discounting the non-distinct entries (which could be analysed as either covert or mandative), less than 6% of mandative clauses were subjunctive ones, whereas 35% of them were covert and 21% were should-mandatives. The remaining examples used other modals such as will or must and so forth. Bear in mind that these are from corpora of spoken English.
I believe that other studies of the mandative in English also support the assertion that the covert mandative is preferred over the subjunctive in British English. However, this observation should be taken with a pinch of salt. The mandative verb/adjective/noun involved may affect a preference for one over the other. Subjunctive mandatives appear to be quite frequent in British English after the verb demand, but very rare indeed after the adjective essential for example. It should also be noted that this may well be affected by the type of genre involved. Mandatives with demand are particularly common in newspapers and legal documents but rarer in informal writing and speech, whereas essential is very common in many different types of spoken and written texts.
Grammar Note from Huddleston & Pullum
Here is the relevant excerpt from the reference grammar The Cambridge Grammar of the English Language by Huddleston & Pullum (2002, pp. 995-996):
"I demand that he leave."
is correct English. The verb "to leave" is in the subjunctive mood. To my ear it sounds better than
"I demand that he leaves."
although people understand either one and many would not notice the difference. The second one sounds like a plausible choice because we say "he leaves" all the time when using the indicative mood. It is used so often in place of the subjunctive that it's not surprising you're confused about them, but it's not the best choice in this situation.
The confusion is compounded by the fact that the third person singular is the only case where the subjunctive is different from the indicative. In the other cases we use the subjunctive and indicative forms interchangeably, so this is an exception to the rule.
For example, in the third person plural we would say,
"They go to the store."
in the indicative, and
"I demand that they go to the store"
in the subjunctive. There's no difference between the two verb forms here.
I strongly demand that John leaves.
I strongly demand that John leave.
The the ghosts of the germanic roots of Old English still stir in English and run through our English brain veins. Thus for some, 'he leaves' is not a candidate for an unreal state. Were he gone, this demand would not be uttered.
That said, how many times does the so called Present Tense refer to an event happening now?
Today is Wednesday. "John leaves on Friday." Obviously, Friday is not 'now' and John has not (yet) left.
The future has not happened. The future is conjecture. Conjecture is conjunctive. Conjunctive is subjudice. Subjudice is subjunctive. In the above sentence, John has not (yet) left.
English, nor any other language, is static. Purists full of Old English blood will tell you, “Only 'he leave' be correct." or, maybe they might say “Only 'he leave' is correct." and the cycle starts again. "Be 'be' or be 'is' correct?"
Therefore, I choose to interpret the meaning despite the words: 'John (vocative), PFO!'