Un simple compliment sur ses fleurs change tout de suite son comportement.

In French, some common adjectives often come before the noun, and simple is one of them when it means something like mere, just, or only.

So:

• un simple compliment = a mere/simple compliment
• un compliment simple would sound unusual here and would suggest a different nuance, more like a compliment that is simple in form

In this sentence, simple means just one small compliment, so putting it before the noun is the natural choice.

Yes. Un compliment in French means a compliment, praise, or a nice remark.

Be careful not to confuse it with English complement (with an e), which means something that completes something else. In French:

• compliment = compliment, praise
• complément = complement, addition, grammatical complement, etc.

So here un simple compliment means a nice remark.

Yes. Here sur means about or concerning.

French sur often literally means on, but it can also mean about, especially when talking about a topic:

• un livre sur Paris = a book about Paris
• un article sur le climat = an article about climate
• un compliment sur ses fleurs = a compliment about his/her flowers

So in this sentence, sur ses fleurs does not mean physically on her flowers. It means the compliment concerns the flowers.

Ses fleurs means his flowers or her flowers, so it shows that the flowers belong to the person being talked about.

Compare:

• ses fleurs = his/her flowers
• les fleurs = the flowers
• des fleurs = some flowers

The sentence is specifically about complimenting that person’s flowers, so the possessive is the natural choice.

It can mean either his or her. French possessive adjectives do not tell you the gender of the owner in this case. They agree with the noun possessed, not with the person who owns it.

Since fleurs is plural, French uses:

• ses = his/her/its

So ses fleurs could mean:

• his flowers
• her flowers

Only context tells you which one.

Because comportement is a masculine singular noun.

French possessives agree with the noun that follows:

• son
  • masculine singular noun
• sa
  • feminine singular noun
• ses
  • plural noun

So:

• son comportement = his/her behavior
• sa réaction = his/her reaction
• ses habitudes = his/her habits

Also, just like ses, son does not tell you whether the owner is male or female. It can mean his or her.

Because the subject is un simple compliment, which is singular.

The verb must agree with the subject:

• Un simple compliment change... = singular subject, singular verb
• De simples compliments changent... = plural subject, plural verb

Even though there are plural flowers in the sentence, ses fleurs is not the subject. It is part of the phrase describing the compliment.

Tout de suite means right away, immediately, or at once.

So it shows that the person’s behavior changes very quickly after the compliment.

A learner might try to understand it word by word as all following or something similar, but that is not how the expression works. It is a fixed phrase:

• Je reviens tout de suite. = I’ll be right back.
• Il a répondu tout de suite. = He answered immediately.

Here it modifies the verb change:

• change tout de suite = changes immediately

In this sentence, comportement means behavior.

Although English has the word comportment, French comportement is usually best translated as:

• behavior
• conduct
• sometimes attitude, depending on context

So son comportement here most naturally means his/her behavior.

Because sur ses fleurs directly describes what the compliment is about.

French can use different prepositions depending on the noun or verb, and here un compliment sur... is a natural way to say a compliment about....

You may also hear other constructions in French depending on style and context, but in this sentence sur works well and clearly links the compliment to the topic of the flowers.

In this sentence, it comes after the verb:

• change tout de suite

That is a very natural placement for adverbs or adverbial expressions of time in French.

So the structure is:

• subject: Un simple compliment sur ses fleurs
• verb: change
• adverbial expression: tout de suite
• object: son comportement

French word order is often a bit more flexible than English, but this version sounds very natural and straightforward.

Yes, absolutely. That is exactly how the grammar works.

The subject is:

• Un simple compliment sur ses fleurs

The thing being changed is:

• son comportement

So the compliment is about the flowers, but what changes is the person’s behavior.

A good way to see the structure is:

• [A simple compliment about his/her flowers] [changes immediately] [his/her behavior].

A few small things:

• un is pronounced with a nasal vowel, not like English un
• simple ends with a pronounced -pl
• compliment in French is not pronounced exactly like English compliment
• ses fleurs has the French eu sound in fleurs
• tout de suite is pronounced smoothly as a fixed expression

Also, many final consonants in French are silent, but you should learn the pronunciation as whole words rather than guessing from spelling.