Jeg synes, at hverdagen bliver lettere, når jeg skriver ting i kalenderen.

Yes. Jeg synes is a very common way to say I think, I feel, or in my opinion.

In this sentence, jeg synes introduces a personal opinion:

• Jeg synes, at ... = I think that ...

A useful comparison:

• jeg synes = opinion / personal impression
• jeg tror = belief / guess / something you think is true
• jeg mener = opinion, often a bit stronger or more deliberate

So here jeg synes fits well because the speaker is talking about how everyday life feels to them.

At here means that and introduces a subordinate clause.

So:

• Jeg synes, at hverdagen bliver lettere
  = I think that everyday life becomes easier

In English, that is often optional:

• I think that ...
• I think ...

In Danish, at is very common after verbs like synes, tror, ved, and siger when a full clause follows.

In everyday Danish, people sometimes leave at out in speech, but learners should be comfortable with it because it is very standard and clear.

Hverdagen is the definite form of hverdag.

• en hverdag = an everyday day / a weekday / an ordinary day
• hverdagen = everyday life / daily life / the everyday routine

In Danish, the definite form is often used where English uses a more general expression. So hverdagen can mean everyday life in a broad, general sense.

That is why:

• hverdagen bliver lettere
  means something like
  everyday life becomes easier

It does not necessarily mean one specific day. It often refers to daily life in general.

Good question. Bliver can do a few different jobs in Danish, but here it clearly means becomes.

• blive = to become

So:

• hverdagen bliver lettere = everyday life becomes easier

This suggests a change: life is easier as a result of writing things in the calendar.

Compare:

• Hverdagen er lettere = everyday life is easier
  → describes a state
• Hverdagen bliver lettere = everyday life becomes easier
  → describes a change

You are also right that blive can be used in passive constructions, but that is not what is happening here.

Lettere is the comparative form of let.

• let = easy / light
• lettere = easier
• lettest = easiest

So:

• hverdagen bliver lettere = everyday life becomes easier

This works just like English easy → easier.

A few examples:

• Det er let. = It is easy.
• Det bliver lettere. = It becomes easier.
• Det er lettest sådan. = It is easiest that way.

Når means when, but in Danish it is often used for things that happen regularly, generally, or whenever something happens.

Here:

• når jeg skriver ting i kalenderen
  = when / whenever I write things in the calendar

That fits because the sentence describes a general pattern, not one single past event.

A useful contrast:

• når = when, whenever, in general / in the future
• da = when, for a specific event in the past
• hvis = if

So:

• Når jeg skriver ting i kalenderen, bliver hverdagen lettere.
  = Whenever I write things in the calendar, everyday life gets easier.

Danish uses the present tense very often for habits, general truths, and repeated actions.

So:

• jeg skriver ting i kalenderen
  literally = I write things in the calendar

In natural English, we often translate that as:

• when I write things in my calendar
• when I write things down in my calendar
• when I put things in my calendar

Danish does not have a separate progressive form like I am writing in the same way English does. The plain present tense often covers both simple and ongoing meanings, depending on context.

So skriver here is perfectly normal for a habitual action.

Ting means things, and here it is being used in a general, indefinite way.

• en ting = a thing
• ting = things
• tingene = the things

So:

• jeg skriver ting i kalenderen
  = I write things in the calendar

This means the speaker is talking generally about writing items, appointments, tasks, and so on into the calendar. They are not referring to specific known things, so tingene would sound different.

Compare:

• Jeg skriver ting i kalenderen. = I write things in the calendar.
• Jeg skriver tingene i kalenderen. = I write the things in the calendar.

The second one sounds much more specific.

In Danish, i kalenderen is the normal expression for putting or writing something in a calendar.

• i kalenderen = in the calendar

This is one of those places where Danish and English prepositions do not match exactly. English speakers might imagine on because you write on a page, but Danish normally says i kalenderen when talking about entering information into a calendar system or planner.

Examples:

• Jeg skriver det i kalenderen. = I write it in the calendar.
• Det står i kalenderen. = It is written in the calendar.

So this is just the standard Danish choice of preposition.

Both are possible, but they mean slightly different things.

• i kalenderen = in the calendar
• i min kalender = in my calendar

Danish often uses the definite form alone when possession is obvious from context. Since the speaker is talking about their own habit, it is already clear that they mean their own calendar.

So i kalenderen sounds natural and not incomplete.

If the speaker wanted to be extra explicit, they could say:

• Jeg synes, at hverdagen bliver lettere, når jeg skriver ting i min kalender.

That is also correct.

Those commas mark the beginning of subordinate clauses.

The sentence has this structure:

• Jeg synes,
• at hverdagen bliver lettere,
• når jeg skriver ting i kalenderen.

In Danish, commas before subordinate clauses are very common. However, there are two accepted comma systems in modern Danish:

1. With start comma

   • Jeg synes, at hverdagen bliver lettere, når jeg skriver ting i kalenderen.

2. Without start comma

   • Jeg synes at hverdagen bliver lettere når jeg skriver ting i kalenderen.

Both can be correct, depending on the writing style being used. The version in your sentence uses the system with start commas.

Because når jeg skriver ting i kalenderen is a subordinate clause, and subordinate clauses do not use the normal main-clause inversion pattern.

In Danish main clauses, the verb often comes in second position:

• Jeg skriver ting i kalenderen.
• I kalenderen skriver jeg ting.

But in subordinate clauses introduced by words like at, når, fordi, hvis, the order is usually more straightforward:

• at hverdagen bliver lettere
• når jeg skriver ting i kalenderen

So:

• når jeg skriver ... = correct
• når skriver jeg ... = not correct here

This is an important pattern in Danish grammar:

• main clause: often verb-second
• subordinate clause: subject usually comes before the verb