With hypothetical denoting an assumption that one condition be true for the sake of argument or for the purpose of investigation, hypothetical meaning in English is expressed with the use of the subjunctive mood. While Romance and other languages have a separate conjugation for verbs in the subjunctive mood, English simply pares the third person singular of its -s ending, except for the vert to be which is used thusly in the subjunctive:
If this be true, then we have made the wrong assumption.
Regular verbs remove the -s from the third person singular:
If a man have patience, he can train a horse.
It is unfortunate that the use of the subjunctive mood is often ignored in English and the present basic form of verbs is used, instead. For, there is a tone to the subjunctive that indicates well the connotations of the use of the subjunctive. Thus, nowadays, is it more common to hear someone say
If a man has patience, he can train horses.