Learn Hardcore Danish: Jeg er sikker på, at planen er god.

You can’t normally say "Jeg er sikker, at planen er god" or just "Jeg er sikker planen er god"; that sounds wrong to native speakers. The natural pattern is:

være sikker på + (noget / at‑sætning)
e.g. Jeg er sikker på det. / Jeg er sikker på, at han kommer.

Can I say "Jeg er sikker at planen er god" like English “I’m sure that the plan is good”?

No. That is not idiomatic Danish.

In Danish you must include "på":

✅ Jeg er sikker på, at planen er god.
❌ Jeg er sikker, at planen er god.
❌ Jeg er sikker at planen er god.

Think of "sikker på" as a fixed combination: to be sure of/about (something).

What is the function of "på" here? Does it literally mean “on”?

Literally, "på" often means “on”, but in this expression it’s mostly idiomatic.

være sikker på (noget) = to be sure of/about (something)

So in:

Jeg er sikker på, at planen er god.

you can think of it as “I am sure about the fact that the plan is good.”
The preposition doesn’t translate directly; it’s just required by "sikker" in this meaning.

What does "at" do in "at planen er god"?

Here "at" is a subordinating conjunction introducing a subordinate clause, similar to English "that":

at planen er god = that the plan is good

So the structure is:

Jeg er sikker på, at [planen er god].
= I am sure that [the plan is good].

Note that "at" can also be the infinitive marker (to in English), as in:

at spise = to eat
at gå = to go

But in this sentence, it is clearly “that”, not the infinitive marker.

Why is there a comma before "at" in "Jeg er sikker på, at planen er god."?

The comma marks the beginning of a subordinate clause introduced by "at".

Traditional Danish comma rules (the grammatical comma) require a comma before most subordinate clauses:

Jeg tror, at han kommer.
Hun sagde, at hun var træt.
Jeg er sikker på, at planen er god.

In modern Danish, there is an officially accepted simplified comma system where this comma may be omitted:

Jeg er sikker på at planen er god. (also correct in that system)

However, many people and most printed texts still use the traditional comma, so you will very often see the comma before "at" in sentences like this.

Why is the word order "at planen er god" and not "at er planen god"?

In Danish, main clauses and subordinate clauses have different word orders.

In main clauses with a verb and a subject, you normally have verb in second position, often before the subject (V2 word order):
- Planen er god. – The plan is good. (Subject–Verb)
- Er planen god? – Is the plan good? (Verb–Subject, because it’s a question)
In subordinate clauses introduced by words like "at", "fordi", "hvis", you usually have Subject–Verb order (no V2):
- at planen er god – that the plan is good
- fordi planen er god – because the plan is good
- hvis planen er god – if the plan is good

So in your sentence:

Jeg er sikker på, at planen er god.

the clause "at planen er god" is subordinate, so it keeps Subject (planen) + Verb (er).

Why is it "planen" and not just "plan" or "en plan"?

Danish usually marks definiteness with a suffix, not a separate word like English "the":

plan = plan (indefinite)
en plan = a plan
planen = the plan

In "at planen er god", we are talking about a specific plan that the speaker and listener already know about, so Danish uses the definite form:

planen = the plan (that we have in mind)

If you said:

at en plan er god = that a plan is good

it would sound like you mean just some plan in general, not a particular one.

Why is the adjective "god" in this form and not "gode" or something else?

"god" is an adjective in its base form. When an adjective comes after the verb (a predicative position), Danish:

does not add the definite ending, even if the noun is definite
does change for plural, but not for singular

For common-gender singular:

Planen er god. – The plan is good.
Bilen er rød. – The car is red.
Manden er træt. – The man is tired.

For plural, you add -e:

Planerne er gode. – The plans are good.
Bilerne er røde. – The cars are red.
Mændene er trætte. – The men are tired.

So with "planen" (singular), you must say "god", not "gode".

Could I say "Jeg er sikker på planen er god" without "at"?

In standard Danish, you should include "at":

✅ Jeg er sikker på, at planen er god.

Leaving out "at":

❌ Jeg er sikker på planen er god.

is generally considered incorrect or at least non‑standard in writing. You often still need that "at" to introduce the subordinate clause after "sikker på".

Spoken Danish can be a bit looser, and you might hear "at" dropped in fast speech, but as a learner you should keep it:

sikker på, at …

What exactly is the difference between "Jeg er sikker på, at planen er god." and "Jeg er sikker på det."?

Both express certainty, but they differ in specificity:

Jeg er sikker på, at planen er god.
– I am sure that the plan is good.
You explicitly state what you are sure about.
Jeg er sikker på det.
– I’m sure of it. / I’m sure about that.
You refer back to something already mentioned (using "det").

So "sikker på det" is more general or context‑dependent, while "sikker på, at planen er god" spells out the content of your certainty.

Why is it "er" and not something like "har" in this sentence?

"er" is the present tense of "være" (to be), and you need "to be" with adjectives:

Planen er god. – The plan is good.
Jeg er træt. – I am tired.
Vi er glade. – We are happy.

"har" is from "have" (to have), used for possession or in perfect tenses:

Jeg har en plan. – I have a plan.
Planen har ændret sig. – The plan has changed.

Since you’re describing a state (good), you must use "er", not "har".

How would I make this stronger or weaker, like “I’m pretty sure …” or “I’m absolutely sure …”?

You can add adverbs before "sikker":

Jeg er lidt sikker på, at planen er god.
– I’m a bit sure that the plan is good. (sounds hesitant, not so common)

More natural modifiers:

Jeg er ret sikker på, at planen er god.
– I’m quite/pretty sure the plan is good.
Jeg er temmelig sikker på, at planen er god.
– I’m fairly sure the plan is good.
Jeg er meget sikker på, at planen er god.
– I’m very sure the plan is good.
Jeg er helt sikker på, at planen er god.
– I’m completely/absolutely sure the plan is good.

So the pattern stays: Jeg er [adverb] sikker på, at …