Først var jeg i tvivl om planen, men nu er jeg sikker på, at den er god.

Because Danish is a V2 language in main clauses: the finite verb usually comes in the second position.

So when Først is placed first, the verb var must come next:

• Først var jeg i tvivl ...

If you said Jeg var først ..., that would normally mean something like I was first, not At first, I was ....

So here Først means at first / initially, and it triggers inversion:

• Først var jeg ...
• Nu er jeg ...

Here først means at first or initially.

In other contexts, først can also mean first, but in this sentence it clearly has the time meaning:

• Først var jeg i tvivl ... = At first I was doubtful ...

If you wanted firstly in a list or argument, Danish would often use other wording, depending on the context.

I tvivl is a fixed expression meaning:

• in doubt
• unsure
• uncertain
• sometimes having doubts

So:

• jeg var i tvivl om planen
  means
• I was unsure about the plan
• I had doubts about the plan

You should learn i tvivl as a chunk, because Danish uses this expression very naturally.

Because the usual pattern is:

• være i tvivl om noget = to be in doubt about something

So:

• i tvivl om planen = unsure about the plan

The preposition om is just the one Danish uses with this expression. It does not translate word-for-word in a perfectly neat way; it is simply the normal construction.

Other examples:

• Jeg er i tvivl om svaret. = I’m unsure about the answer.
• Hun var i tvivl om, hvad hun skulle gøre. = She was unsure what she should do.

Because the sentence contrasts the past with the present:

• var = was (past tense)
• er = am / is (present tense)

So the meaning is:

• At first I was unsure about the plan, but now I am sure ...

This change in tense matches the time words:

• Først → earlier situation
• nu → current situation

You can say jeg er sikker, but when you want to say what you are sure about, Danish normally uses:

• være sikker på ... = to be sure that / sure about ...

So:

• jeg er sikker på, at den er god
  means
• I am sure that it is good

The preposition på is part of the standard pattern.

Examples:

• Jeg er sikker på svaret. = I’m sure about the answer.
• Jeg er sikker på, at hun kommer. = I’m sure that she is coming.

Because at den er god is a subordinate clause introduced by at = that.

In standard Danish writing, a comma is often placed before a subordinate clause:

• ... sikker på, at den er god

This is very normal in written Danish.

So the structure is:

• main clause: men nu er jeg sikker på
• subordinate clause: at den er god

Because planen comes from en plan, which is a common gender noun in Danish.

Common gender nouns are referred to with den:

• en plan → den

Neuter nouns take det:

• et hus → det

So:

• planen = the plan
• den = it (referring to planen)

That is why the sentence says:

• at den er god

not

• at det er god

Because subordinate clauses in Danish usually have subject before the verb.

So in the subordinate clause:

• at den er god
• that it is good

The order is:

1. at
2. subject: den
3. verb: er
4. complement: god

This is different from the main clause pattern, where Danish often puts the finite verb in second position.

Compare:

Main clause

• Nu er jeg sikker ...

Subordinate clause

• ... at den er god

This is one of the big word-order differences learners need to notice in Danish.

Because god agrees with the noun it refers to.

Here den refers to planen:

• en plan → common gender singular

For a common gender singular noun, the adjective is usually the basic form:

• en god plan

So when referring back to planen, we get:

• den er god

You would use godt with a neuter singular noun:

• et godt forslag = a good proposal
• det er godt = it is good

Men means but, and nu means now.

Together they create a contrast:

• earlier: Først var jeg i tvivl ...
• now: men nu er jeg sikker ...

So the sentence is built around a clear change in attitude:

1. first, uncertainty
2. now, confidence

Also, because nu comes before the verb in the second clause, Danish again uses inversion:

• men nu er jeg sikker ...

not

• men nu jeg er sikker ...

They are similar, but not always identical.

• i tvivl often means having doubts / being uncertain
• usikker can mean uncertain, but it can also mean insecure depending on context

In this sentence, i tvivl om planen is very natural because it specifically expresses doubt about something.

You could say:

• Jeg var usikker på planen

but jeg var i tvivl om planen sounds especially idiomatic for I had doubts about the plan.

A useful way is to memorize it in chunks:

• Først var jeg ... = At first I was ...
• i tvivl om ... = unsure about ...
• men nu er jeg sikker på ... = but now I am sure that / sure about ...
• at den er god = that it is good

So the full pattern is:

• Først var jeg i tvivl om X, men nu er jeg sikker på, at den er Y.

You can then swap in new words:

• Først var jeg i tvivl om ideen, men nu er jeg sikker på, at den er rigtig.
• Først var jeg i tvivl om filmen, men nu er jeg sikker på, at den er god.

That makes the grammar much easier to reuse naturally.