Min søster er ikke bare ærlig, men også meget tålmodig, når vi diskuterer svære ting.

Because min already functions as the determiner, just like my in English.

In Danish, you normally do not combine a possessive like min with a definite article. So:

• min søster = my sister
• søsteren = the sister

So min søsteren is not correct standard Danish.

Er is the present tense of at være = to be.

Unlike English, Danish verbs do not change according to the subject in the present tense. So you get:

• jeg er = I am
• du er = you are
• hun er = she is
• vi er = we are

So Min søster er ... is correct simply because er is the present-tense form for everyone.

This is a very common Danish pattern meaning not only ..., but also ...

So:

• ikke bare ærlig = not only honest
• men også meget tålmodig = but also very patient

It works much like English. The structure links two qualities and gives emphasis.

A close alternative is:

• ikke kun ..., men også ...

Both are natural, but ikke bare ..., men også ... is very common in everyday Danish.

Because they are predicative adjectives: they come after er and describe the subject.

In Danish, predicative adjectives still agree with the subject, but here the subject is søster, which is a common-gender singular noun. For that form, the adjective usually appears in its basic form:

• Min søster er ærlig
• Min søster er tålmodig

Compare:

• Et barn er ærligt = a child is honest
• De er ærlige = they are honest

So the form depends on the noun being described.

Because meget here is an adverb meaning very.

It modifies the adjective tålmodig, just like English very modifies patient:

• meget tålmodig = very patient

Megen is different. It is a determiner meaning something like much/a lot of, and it is not used this way before an adjective.

So:

• meget tålmodig = correct
• megen tålmodig = incorrect

Because svære is an adjective placed before a plural noun.

In Danish, adjectives before nouns usually take -e in the plural:

• en svær ting = a difficult thing
• svære ting = difficult things

So the -e tells you that ting is plural here.

Here it is plural.

The tricky part is that ting has the same form in:

• indefinite singular: en ting = a thing
• indefinite plural: ting = things

So you have to look at the surrounding words. In this sentence, svære shows that the noun is plural:

• svære ting = difficult things

Compare:

• en svær ting = a difficult thing
• svære ting = difficult things
• tingen = the thing
• tingene = the things

Here når means when in a general or repeated sense: whenever / when we discuss difficult things.

Danish often uses:

• når for repeated, general, or future situations
• da for one specific situation in the past

So:

• Når vi diskuterer svære ting = when/whenever we discuss difficult things
• Da vi diskuterede det i går = when we discussed it yesterday

In this sentence, når fits because the meaning is general, not one single past event.

Because når introduces a subordinate clause, and subordinate clauses in Danish usually have normal subject + verb order:

• vi diskuterer = we discuss

So:

• når vi diskuterer svære ting = when we discuss difficult things

Danish main clauses often use verb-second word order, but subordinate clauses do not work the same way.

Compare:

• Vi diskuterer svære ting. = main clause
• ..., når vi diskuterer svære ting. = subordinate clause

Because når vi diskuterer svære ting is a subordinate clause, and many Danish writers put a comma before subordinate clauses.

However, this is one of those places where Danish punctuation can vary depending on comma style. In modern Danish, you may see both:

• ..., meget tålmodig, når vi diskuterer svære ting.
• ..., meget tålmodig når vi diskuterer svære ting.

So the comma before når is common and correct in one standard comma system, but you may also see the sentence without it.

Usually diskutere means discuss.

It can sometimes involve disagreement, but it is usually more neutral than English argue. If you want the sense of quarrel or fight verbally, Danish often uses other verbs, such as skændes.

So in this sentence:

• vi diskuterer svære ting most naturally means we discuss difficult things

It does not automatically suggest an angry argument.