Learn Hardcore Danish: Jeg er sikker på, at min ven kommer i aften. - I am sure that my friend is coming tonight.

If you just say “Jeg er sikker, at …”, it sounds wrong to Danish ears.
The preposition “på” is part of the fixed pattern “sikker på, at …” and cannot be left out in standard Danish.

What does “på” mean here? Could I use another preposition?

Literally, “på” means “on”, but in this expression it doesn’t keep a clear literal meaning. It’s just the preposition required by “sikker” in this context.

correct: Jeg er sikker på, at min ven kommer.
wrong: Jeg er sikker om, at min ven kommer.
wrong: Jeg er sikker af, at min ven kommer.

So: you can’t really replace “på” with another preposition here; it’s a fixed combination.

Can I say “Jeg er sikker, at min ven kommer i aften” without “på”?

No, that is considered incorrect in Danish.

You need:

either “Jeg er sikker på, at min ven kommer i aften.”
or a different verb that doesn’t need “på”, e.g.
- “Jeg ved, at min ven kommer i aften.” (I know that…)
- “Jeg tror, at min ven kommer i aften.” (I think that…)

With “sikker” in this meaning, Danish wants “på”: “sikker på (at …)”.

Why is there a comma before “at”? Is it always required?

“At min ven kommer i aften” is a subordinate clause (“that my friend is coming tonight”).

Traditionally, Danish always put a comma before such clauses:

Traditional (very common):
Jeg er sikker på, at min ven kommer i aften.

Since a spelling reform, it is also allowed to omit that comma:

Also correct (newer system):
Jeg er sikker på at min ven kommer i aften.

So:

Both versions are correct.
Most Danes still keep the comma in writing.
As a learner, it’s safe and common to use the comma before “at” when it introduces a clause.

Is “at” always necessary in this kind of sentence?

Here, yes: with “sikker på”, you basically always say “på, at …”:

Jeg er sikker på, at min ven kommer i aften.

In Danish, “at” can sometimes be dropped after some verbs in informal speech (e.g. Jeg tror (at) han kommer), but:

After “sikker på”, dropping “at” is not normal; it sounds wrong or incomplete.
So you should keep “at” here.

Why is “kommer” in the present tense if it refers to the future (tonight)?

Danish often uses the present tense to talk about the near future, especially for planned or expected events:

Min ven kommer i aften.
= My friend is coming tonight / will come tonight.

You can use future-type verbs, but they add nuance:

Min ven skal komme i aften.
= He is supposed to come tonight (plan/arrangement).
Min ven vil komme i aften.
= He wants/intends to come tonight (intention, willingness).

In your sentence, the simple present “kommer” is the most natural way to say this.

Why is the word order “min ven kommer i aften” and not “kommer min ven i aften”?

Because this part is a subordinate clause introduced by “at”:

at min ven kommer i aften

In Danish:

In main clauses, the verb is in 2nd position (V2):
- Min ven kommer i aften.
- I aften kommer min ven.
In subordinate clauses, the normal order is Subject – Verb – (other elements):
- at min ven kommer i aften (subject min ven before verb kommer)

So “at min ven kommer i aften” is correct;
“at kommer min ven i aften” would be ungrammatical.

Can I move “i aften” to another place in the sentence?

Inside this subordinate clause, the natural place is after the verb and object/subject:

Jeg er sikker på, at min ven kommer i aften. ✅
Jeg er sikker på, at i aften min ven kommer. ❌ (sounds wrong)
Jeg er sikker på, at min ven i aften kommer. ❌ (also wrong/very odd)

You can move “i aften” if you move the whole clause:

I aften er jeg sikker på, at min ven kommer.
= Tonight I am sure that my friend is coming.

But inside the “at …”-clause, keep “i aften” at the end.

What is the difference between “i aften”, “i nat”, and “i aftes”?

They all refer to time, but not the same:

i aften – this evening / tonight (the coming evening, roughly until bedtime).
i nat – tonight in the night-time sense (the coming night, when it’s dark and people sleep).
i aftes – last night / yesterday evening (the evening that has already passed).

So your sentence is about later today in the evening, not the night or yesterday.

Why is it “min ven” and not “mit ven”?

Danish has two grammatical genders:

common gender (n-words) → use “en” in the indefinite: en ven
neuter gender (t-words) → use “et”: et hus (a house)

The possessive “my” changes with gender and number:

min – common gender singular: min ven (my friend), min bil (my car)
mit – neuter singular: mit hus (my house)
mine – all plurals: mine venner (my friends)

Since “ven” is common gender (en ven), it must be “min ven”, not “mit ven”.

Why don’t we say “min vennen” for “my friend”?

In Danish, you don’t combine a possessive like “min” with the definite ending -en/-et on the noun.

correct: vennen = the friend
correct: min ven = my friend
wrong: min vennen ✗

So:

“vennen” already means “the friend” by itself.
When you use “min”, the noun must stay indefinite: “min ven”.

Could I say “Jeg er sikker på, at min ven også kommer i aften” and where should “også” go?

Yes, you can add “også” (also / too), but its position matters.

Natural options:

Jeg er sikker på, at min ven også kommer i aften.
= I’m sure that my friend will also come tonight.
Jeg er sikker på, at min ven kommer i aften også.
= very marked/awkward in Danish; normally avoid this.

In subordinate clauses, adverbs like “også” usually go after the subject and before the verb or right after the verb phrase, depending on emphasis. Here “min ven også kommer i aften” is the normal word order.