[Grammar] 'Before he enter on the execution of his office […]'

[Pascal13]

This is the pretext of the oath of office of the President of the United States. I wondered why they use the structure 'he' plus infinitive and not 'he' plus s as it is usually used. I've already figured out that it could be a usage of the subjunctive mood, but I don't know why it should be used in this situation.
Thank you for your help.

 

[GoesStation]

You mean the context. Here's Article II, Section One, Clause 8 of the United States Constitution:

Before he enter on the Execution of his Office, he shall take the following Oath or Affirmation:—"I do solemnly swear (or affirm) that I will faithfully execute the Office of President of the United States, and will to the best of my ability, preserve, protect and defend the Constitution of the United States."

The verb enter​ is an obsolete instance of the subjunctive mood. Nowadays we'd write that in the indicative.

 

[Pascal13]

Thank you very much !

Yes, context would suit better here, I just wanted to point out that it's before the actual inauguration oath.
I see ! Hm so why did the use the subjunctive mood here ? I know that you can use it if something is contrary to a fact like in 'If I were you, I would ..' and I'm also aware that more and more natives tend to stop using it.

 

[GoesStation]

Yes, context would suit better here, I just wanted to point out that it's before the actual inauguration oath.
I see ! Hm so why did the use the subjunctive mood here ? I know that you can use it if something is contrary to a fact like in 'If I were you, I would ..' and I'm also aware that more and more natives tend to stop using it.

The subjunctive is used less in modern English than it used to be. American English retains a number of features that have faded out in the British variety, including more-frequent use of the subjunctive. We no longer use it after before, though, and I don't know enough about late Eighteenth-century American English to know why the writers of the Constitution found it natural there.

Don't put a space before an ellipsis, an exclamation mark, or any other punctuation that ends a sentence.

An ellipsis requires three periods. Write "If I were you, I would..." with three dots connected to the last letter of the final word.