Ce n’était qu’un malentendu : personne n’avait déplacé ses pots exprès.

Here ne ... que means only or just.

So:

• Ce n’était qu’un malentendu = It was only / just a misunderstanding

This is a very common French structure. Even though it uses ne, it is not a full negative like not in English. It is a restrictive structure.

Compare:

• Il ne mange que du pain = He only eats bread
• Ce n’était qu’un malentendu = It was just a misunderstanding

So the idea is: nothing more than a misunderstanding.

Because the ne comes between ce and était.

French often shortens ce to c’ before a vowel sound:

• c’est
• c’était
• ce sera stays ce sera because sera begins with a consonant sound

But when ne is inserted, the word directly after ce is now ne, which starts with a consonant sound, so there is no elision:

• Ce n’est pas vrai
• Ce n’était qu’un malentendu

So:

• c’était = no ne in between
• ce n’était = ne blocks the contraction

The imparfait (n’était) is used here because the sentence is presenting a state or situation in the past, not a single completed action.

• être often appears in the imperfect when describing what something was
• Ce n’était qu’un malentendu gives background or explanation

It has the feel of:

• It was simply a misunderstanding
• The situation turned out to be just a misunderstanding

If you used ça a été or another completed form, it would sound different and less natural here. The imperfect is the normal choice for describing the nature of a past situation.

In standard French, when personne means no one / nobody, it usually works with ne.

So:

• Personne n’avait déplacé... = Nobody had moved...

This is the standard negative pattern.

Examples:

• Personne ne vient = No one is coming
• Personne n’a compris = Nobody understood

In informal spoken French, people often drop ne:

• Personne avait déplacé...

But that is colloquial speech. In correct written French, ne is expected.

Because personne here means no one / nobody, which is grammatically singular.

So French says:

• Personne n’avait déplacé... not
• Personne n’avaient déplacé...

Even though personne refers to zero people, it is treated like a singular subject, just like English no one has rather than no one have.

Because this is the plus-que-parfait: avait déplacé = had moved.

The plus-que-parfait is used for an action that happened before another moment in the past.

Here the sentence is looking back at a past situation and saying that, at that point, nobody had moved the pots on purpose. In other words, the moving would have happened earlier than the moment when people realized it was a misunderstanding.

So the timeline is roughly:

1. The supposed moving of the pots
2. The later realization: it was just a misunderstanding

That is why French uses had moved, not simply moved.

Because with avoir, the past participle normally agrees only if the direct object comes before the verb.

Here we have:

• avait déplacé ses pots

The direct object ses pots comes after the verb, so there is no agreement.

That is why it stays:

• déplacé not
• déplacés

Compare:

• Il a déplacé les pots → no agreement
• Les pots qu’il a déplacés → agreement, because les pots comes before the participle

This is a classic French grammar rule.

Ses means his, her, or sometimes their, depending on context.

A very important point: ses agrees with the thing owned, not with the owner.

Because pots is plural, French uses:

• son for one masculine noun
• sa for one feminine noun
• ses for plural nouns

So:

• son pot = his/her pot
• ses pots = his/her pots

By itself, ses does not tell you whether the owner is male or female. Context has to tell you that.

Exprès means deliberately, intentionally, or on purpose.

So:

• déplacé ses pots exprès = moved his/her pots on purpose

Putting exprès at the end is very natural in French. It modifies the whole action and gives emphasis to the idea of intention.

Compare:

• Il l’a fait exprès = He did it on purpose
• Personne n’avait déplacé ses pots exprès = Nobody had moved the pots intentionally

French also has the expression faire exprès, but exprès can also be used directly with many verbs, as it is here.

The colon introduces an explanation or clarification.

So the structure is:

• Ce n’était qu’un malentendu
  :
• personne n’avait déplacé ses pots exprès

The second part explains the first part:

• It was just a misunderstanding: nobody had moved the pots on purpose.

In English, you could also use:

• a colon
• a dash
• or even a full stop

French uses the colon quite naturally for this kind of explanatory relationship.

Yes, malentendu is the normal noun for misunderstanding.

It is a masculine noun:

• un malentendu
• des malentendus

It is built historically from the idea of something badly understood, but for learners it is best treated as a standard vocabulary item meaning misunderstanding.

Common examples:

• C’était un malentendu = It was a misunderstanding
• Il y a eu un malentendu = There was a misunderstanding

So in this sentence, it is a very natural and idiomatic choice.