Hon måste ha lagt originalet i fel mapp.

Because måste is not marking past time by itself here. In Hon måste ha lagt ..., it means a present deduction about a past action:

• måste = must
• ha lagt = have put/placed

So the whole structure means she must have put ...

This is very common in Swedish. Even though the action happened in the past, the speaker is making the conclusion now, so Swedish uses måste + ha + supine.

It is the Swedish equivalent of must have put or must have placed.

Structure:

• måste = must
• ha = have
• lagt = put/placed

So:

• Hon måste ha lagt originalet i fel mapp.
• She must have put the original in the wrong folder.

This expresses probability or deduction, not obligation. The speaker is guessing what happened based on evidence.

Because after ha, Swedish uses the supine form of the verb, not the infinitive or the simple past.

For lägga:

• infinitive: lägga
• present: lägger
• past: lade
• supine: lagt

So:

• Hon lade originalet i fel mapp. = She put the original in the wrong folder.
  This is a plain past statement.

• Hon måste ha lagt originalet i fel mapp. = She must have put the original in the wrong folder.
  Here lagt is required because it follows ha.

The base form is lägga, which usually means to put, to place, or sometimes to lay.

In this sentence, lägga is best understood as put/place:

• lägga något i en mapp = put something in a folder

So lagt is just the form used after ha.

Because originalet is the definite singular form: the original.

The noun is:

• ett original = an original
• originalet = the original

The ending -et is the normal definite ending for many ett nouns.

So:

• Hon lade originalet ... = She put the original ...
• Hon lade ett original ... = She put an original ...

Here fel means wrong.

In phrases like fel mapp, fel plats, fel person, Swedish often uses fel as an indeclinable modifier, so it does not change for gender or number.

Examples:

• fel mapp = wrong folder
• fel plats = wrong place
• fel personer = wrong people

So fel stays fel, not something like fela or felt.

Because Swedish often uses fel + noun without an article in this kind of expression.

So i fel mapp is the natural way to say in the wrong folder.

Compare a few common patterns:

• på fel plats = in the wrong place
• vid fel tidpunkt = at the wrong time
• i fel mapp = in the wrong folder

This is more idiomatic than trying to build it word-for-word from English.

Because after the verb, Swedish commonly puts the direct object before a place expression.

So the order is:

• lagt = put
• originalet = the original
• i fel mapp = in the wrong folder

That gives:

• lagt originalet i fel mapp

This is the most natural order in a neutral sentence. It matches English fairly closely:

• put the original in the wrong folder

Here it is expressing a logical conclusion, not obligation.

So the meaning is:

• She must have put the original in the wrong folder.

That means something like:

• I conclude that she probably did that.
• That seems to be the explanation.

It does not mean that she was required to put it there.

You would remove måste ha and use the simple past:

• Hon lade originalet i fel mapp.

That means:

• She put the original in the wrong folder.

So the difference is:

• Hon lade ... = this is stated as a fact
• Hon måste ha lagt ... = this is an inference or assumption

Be careful here, because Swedish and English do not match perfectly.

If you want She cannot have put the original in the wrong folder, Swedish normally says:

• Hon kan inte ha lagt originalet i fel mapp.

That is the natural negative deduction.

But:

• Hon måste inte ha lagt ...

usually means She did not have to put ..., not She must not have put ...

So for negative logical deduction, kan inte ha lagt is usually the best choice.

It can mean either one. Mapp is used for both:

• a paper/document folder
• a digital folder on a computer

Usually the context tells you which one is meant. In this sentence, either reading is possible unless the wider context makes it clear.