Die Sekretärin sagt, ich solle die unterschriebenen Unterlagen in die blaue Mappe legen.

solle is the Konjunktiv I form of sollen for ich.

German often uses Konjunktiv I in reported speech. It shows that the speaker is reporting what someone else said, rather than stating it directly as their own fact or opinion.

So:

• direct instruction: Sie sollen die Unterlagen in die Mappe legen.
• reported version: Die Sekretärin sagt, ich solle die Unterlagen in die Mappe legen.

In everyday spoken German, many people would also say ich soll instead of ich solle. But ich solle is the more textbook, formal reported-speech form.

Not exactly.

Here ich solle does not mainly express the speaker’s own opinion, like I think I should. Instead, it means something like:

• she says I am supposed to ...
• she says I should ...
• she says I am to ...

So the idea is reported obligation/instruction, not personal reflection.

German can report speech in more than one way.

You can say:

• Die Sekretärin sagt, ich solle ...
• Die Sekretärin sagt, dass ich ... soll
• Die Sekretärin sagt, dass ich ... solle

All three are possible, but they sound a little different:

• without dass
  • Konjunktiv I often sounds more formal, written, or journalistic
• with dass is very common in everyday German
• with dass
  • Konjunktiv I is possible, but usually sounds even more formal

So the sentence is normal, but it is not the most casual everyday phrasing.

Because sollen is a modal verb, and German modal verbs usually work with another verb in the infinitive.

In a clause like this, German creates a kind of verb bracket:

• finite verb near the beginning: solle
• infinitive at the end: legen

So:

• ich solle ... legen

This is the normal pattern with modal verbs such as:

• können
• müssen
• dürfen
• wollen
• sollen

Because after a modal verb, German uses the bare infinitive.

So you say:

• ich soll legen
• ich kann kommen
• ich muss gehen

not:

• ich soll zu legen
• ich kann zu kommen
• ich muss zu gehen

So in this sentence, solle correctly takes legen without zu.

Because in is a two-way preposition.

With two-way prepositions, German uses:

• accusative for movement toward / into a place
• dative for location in a place

Here the papers are being moved into the folder, so German uses accusative:

• in die blaue Mappe

Compare:

• Ich lege die Unterlagen in die blaue Mappe.
  movement into the folder

• Die Unterlagen liegen in der blauen Mappe.
  location inside the folder

That is one of the most important accusative/dative contrasts in German.

That is because the two noun phrases have different gender/number/case combinations.

1. die unterschriebenen Unterlagen

• Unterlagen = plural
• here = accusative plural
• article = die
• after a definite article in accusative plural, the adjective usually takes -en

So: die unterschriebenen Unterlagen

2. die blaue Mappe

• Mappe = feminine singular
• here = accusative singular
• article = die
• after a definite article in feminine accusative singular, the adjective usually takes -e

So: die blaue Mappe

This is part of the normal weak adjective ending pattern.

Here is the case breakdown:

• Die Sekretärin = nominative
  subject of sagt

• ich = nominative
  subject of solle

• die unterschriebenen Unterlagen = accusative
  direct object of legen

• in die blaue Mappe = accusative after in
  because it expresses movement into the folder

So the main things to notice are:

• nominative for the people doing the actions
• accusative for the thing being placed
• accusative again after in because of direction

It comes from the verb unterschreiben.

Its past participle is:

• unterschrieben = signed

In this sentence, that participle is being used like an adjective in front of a noun:

• die unterschriebenen Unterlagen = the signed documents

When a participle is used as an adjective in German, it takes normal adjective endings, which is why you see unterschriebenen rather than just unterschrieben.

Because the sentence contains another clause after sagt:

• main clause: Die Sekretärin sagt
• reported clause: ich solle die unterschriebenen Unterlagen in die blaue Mappe legen

German uses commas very regularly to separate clauses, and this comma is required.

So the comma is not optional punctuation style here; it is part of standard German grammar.

Yes, it is grammatical and natural, but it sounds a bit formal or careful because of ich solle.

In everyday conversation, many speakers would more likely say:

• Die Sekretärin sagt, ich soll die unterschriebenen Unterlagen in die blaue Mappe legen.
• Die Sekretärin sagt, dass ich die unterschriebenen Unterlagen in die blaue Mappe legen soll.

The version with ich solle is especially common in:

• formal writing
• reported speech
• news style
• careful narration

So a learner should understand it well, but should also know that everyday spoken German often uses simpler alternatives.

legen is the more precise and natural verb here.

German often distinguishes placement verbs more specifically than English:

• legen = lay something down
• stellen = stand something upright
• setzen = set someone/something into a sitting position

Since documents are typically placed flat into a folder, legen is a very natural choice.

You may hear tun in casual speech:

• die Unterlagen in die Mappe tun

That is understandable, but it is less precise and usually less polished than legen.