Min mor minder mig om at tage min paraply med, hvis det regner.

This is the normal Danish pattern for to remind someone to do something:

• at minde nogen om noget = to remind someone about something
• at minde nogen om at gøre noget = to remind someone to do something

So:

• Min mor minder mig om at tage min paraply med
  = My mother reminds me to take my umbrella with me

Here:

• minder = reminds
• mig = me
• om at tage... = to take...

Because mig is the object form of jeg.

• jeg = I
• mig = me

In this sentence:

• Min mor is the subject
• mig is the person being reminded, so it has to be the object form

Compare:

• Jeg tager min paraply med = I take my umbrella with me
• Min mor minder mig om... = My mother reminds me...

Because at marks the infinitive, like English to.

• at tage = to take

After minde nogen om at..., Danish normally uses at + infinitive:

• minde mig om at ringe = remind me to call
• minde mig om at skrive = remind me to write
• minde mig om at tage min paraply med = remind me to take my umbrella with me

Because tage med works like a separable verb or a phrasal verb.

• tage med = take along / bring

When there is an object, it usually goes between the verb and med:

• tage paraplyen med
• tage min paraply med
• tage børnene med

So med stays at the end of that verb phrase.

A useful way to think about it is:

• tage ... med = take ... along

It can often match either English take along or bring, depending on context.

In this sentence, tage min paraply med means something like:

• take my umbrella with me
• bring my umbrella along

So the Danish expression is broader than a strict English take/bring distinction.

Because Danish usually uses a possessive instead of the definite ending when you say whose thing it is.

• min paraply = my umbrella
• paraplyen = the umbrella

You normally do not combine them:

• min paraply = correct
• min paraplyen = incorrect

So if the speaker means my umbrella, min paraply is the right form.

This is a very important Danish point.

• sin/sit/sine refers back to the subject of the clause
• The subject here is Min mor

So:

• sin paraply would mean her own umbrella — the mother's umbrella
• min paraply means my umbrella — the speaker's umbrella

That is why min is correct here.

Because hvis means if, while når means when.

Use hvis for something conditional or uncertain:

• hvis det regner = if it rains / if it is raining

Use når for something expected or regular:

• når det regner = when it rains / whenever it rains

So this sentence means the reminder applies under a condition: if it rains.

Det is a dummy subject, just like it in English weather expressions.

• det regner = it is raining / it rains
• det sner = it is snowing
• det blæser = it is windy / it blows

The det does not refer to a specific thing. It is simply required by the grammar.

It can mean either, depending on context.

Danish present tense often covers both:

• it rains
• it is raining

So hvis det regner can be understood as:

• if it rains
• if it is raining

In many cases, Danish does not need a separate form like English is raining.

Because hvis det regner is a subordinate clause.

The main clause is:

• Min mor minder mig om at tage min paraply med

The subordinate clause is:

• hvis det regner

The comma helps mark that extra clause.

A small note: comma use before subordinate clauses can vary somewhat depending on the Danish comma system being followed, but the comma in your sentence is completely normal.

Because after a subordinating conjunction like hvis, Danish normally uses subject + verb order.

So:

• hvis det regner = correct
• hvis regner det = incorrect

This is different from main-clause inversion patterns in Danish. In a subordinate clause, the structure is usually more straightforward:

• hvis + subject + verb

Because Danish separates the ideas the same way English does:

• huske = remember
• minde nogen om = remind someone

So:

• Jeg husker min paraply = I remember my umbrella
• Min mor minder mig om min paraply = My mother reminds me about my umbrella

You cannot normally use huske to mean remind. For that, Danish uses minde.