Quando il fascicolo sarà completo, userò la spillatrice, aggiungerò altre graffette e metterò tutto nella cartellina.

This is a very common difference between English and Italian.

In Italian, when you talk about a future event after quando, you often use the future tense:

• Quando il fascicolo sarà completo...
• literally: When the file will be complete...
• natural English: When the file is complete...

So Italian is being more explicit than English about the future time reference.

Not here. In this sentence, quando introduces a real, expected future event, so the normal choice is the future indicative:

• Quando il fascicolo sarà completo...

Using sia would sound wrong in this context.
The subjunctive after quando is much less common and usually belongs to special, literary, or less direct structures. For a straightforward sentence like this, sarà is the standard form.

Because all three actions happen after the file becomes complete. The sentence lays out a sequence of future actions:

1. the file will be complete
2. I will use the stapler
3. I will add more staples/clips
4. I will put everything in the folder

Italian keeps all of those actions in the future to match the timeline.

They are all forms of the simple future.

Here is what is happening:

• essere → sarà = it will be
• usare → userò = I will use
• aggiungere → aggiungerò = I will add
• mettere → metterò = I will put

A few useful notes:

• For -are verbs, the a changes to e in the future:
  • usare → userò, not usarò
• For -ere and -ire verbs, you usually add the future endings to the stem:
  • aggiungere → aggiungerò
  • mettere → metterò

The -ò ending means I will.

Because Italian normally drops subject pronouns when the verb form already shows who the subject is.

• userò already means I will use
• aggiungerò already means I will add
• metterò already means I will put

You could say io userò if you want emphasis, contrast, or clarity, but it is not necessary.

Italian uses articles more often than English does.

In this sentence, the speaker is referring to specific, identifiable office items:

• il fascicolo = the file / case file
• la spillatrice = the stapler
• la cartellina = the folder

Even when English might sound less article-heavy, Italian often prefers the definite article.

Yes, but they are not the same thing.

• fascicolo usually means a file, case file, or bundle of documents
• cartellina usually means a folder, often a thin paper or cardboard one used to hold documents

So the idea is roughly:

• first there is the set of papers/documents
• then everything gets placed into a folder

Because completo agrees with il fascicolo, which is:

• masculine
• singular

So the adjective must also be masculine singular:

• il fascicolo completo

If the noun were feminine singular, it would be completa.
If it were masculine plural, it would be completi.

Yes, più graffette is possible, but the nuance is a little different.

• altre graffette = other / additional staples or clips
• più graffette = more staples or clips

In many contexts they are close in meaning.
Altre often sounds like you are adding some extra ones to what is already there.

Also notice there is no article before altre graffette. That is normal for an indefinite plural idea like some more staples/clips.

That is a very good question.

In everyday Italian, graffetta very often means paper clip.
If you mean a staple used in a stapler, many speakers would prefer:

• punti
• punti metallici
• punti per cucitrice

So:

• aggiungerò altre graffette is understandable
• but if you specifically mean staples, a more idiomatic choice may be aggiungerò altri punti

This depends somewhat on region and context, but it is a useful distinction to know.

Here tutto means everything as a single whole.

It is being used in a general, neutral way to refer to all the materials together:

• metterò tutto nella cartellina = I’ll put everything in the folder

It is not agreeing with cartellina.
If you were referring back to a specific feminine noun, then tutta might be possible in a different sentence.

Because in + la contracts to nella.

This is one of the normal Italian preposition + article combinations:

• in + la = nella
• in + il = nel
• in + i = nei
• in + le = nelle

So:

• nella cartellina = in the folder

Because the sentence begins with a subordinate time clause:

• Quando il fascicolo sarà completo, ...

In Italian, when this kind of clause comes first, it is very common to separate it from the main clause with a comma.

So the comma helps mark the structure:

• time clause first
• main actions after it

For a future situation, sarà completo is the safest and most standard choice.

• Quando il fascicolo sarà completo... = When the file is complete / will be complete...

Using the present tense here is generally less natural if you are talking about one specific future moment. Italian often prefers the future where English would use the present.

So for learners, the best model is definitely:

• Quando ... sarà ...
• ... userò / aggiungerò / metterò