Is it correct if I complete a sentence with "Before"?

I'm asking for the correct format of this sentence:

My friend: "Can we go for a walk?"

Me: "Sorry, but I have a python course and I need to complete it before".

Is it correct to say this?

Solution 1:

It is correct to end some sentences with "before", for example:

I just typed the homework for my Python course with my toes. I've never done that before!

The example you gave might be understood, but would be more correctly worded as:

Sorry, but I have a python course and I need to complete it beforehand.

or

Sorry, but I have a python course and I need to complete it first.

Solution 2:

You can complete a sentence with before, but only if you’re using before adverbially, where it means, roughly, prior to now:

I’ve never seen that before.
The economy will grow at the same rate as before.

You can see more examples from the Corpus of Contemporary American English — before .

But your before is a conjunction, and you’ve left off the other half of what it’s “conjuncting”:

I have a Python course, and I need to complete it before I can go for a walk.

You can find more examples of before as an adverb, preposition, and conjunction at Collins — before.

What you need is an adverb, like first or beforehand.

Solution 3:

Although 'before' can be used without a prepositional complement (Lexico uses the traditional classification adverb here while CGEL doubtless considers this an intransitive preposition), the other event in question needs to be specified, and will be a notional noun phrase:

before: During the period of time preceding a particular [overtly mentioned, or understood] event or time.

• My playing days had ended six years before [eg the time just mentioned]

the implied 'completer' here ('I come for a walk', etc) is too complex to qualify. It is an independent clause, and 'uncompleted conjunctions' are not recognised.

Some more examples, one unacceptable in standard English:

• After the Great War, women had far more influence in politics than before.
• "Are you still playing cricket now you're married?" ... "No, I have far less time than I had before."
• "I've never driven a tractor before." (ie before now or the time mentioned)
• *"Can we go for a walk?" ... "Sorry, but I have a python course and I need to complete it before." [independent clause missing; ungrammatical.]
• "Can we go for a walk?" ... "Sorry, but I have a python course and I need to complete it before I can come with you."

Solution 4:

Before can be used at the end of a sentence.

I have not seen him before. (present perfect tense)
Have you been to London before? (present perfect tense)
I had not seen a tiger before. (past perfect tense)

I have seen him before. (present perfect tense)
I saw him three years ago. (simple past tense)
Before is used in the perfect tense while ago is used with the simple past tense.