Il faut tailler ce rosier avant l’été pour qu’il fleurisse mieux.

Il faut means it is necessary, one must, or more naturally in English, you need to / you have to.

In this sentence, Il faut tailler ce rosier... means:

This rosebush needs to be pruned...
or
You need to prune this rosebush...

It is a very common French structure:

• Il faut partir. = We have to leave.
• Il faut étudier. = You have to study.

It is called an impersonal expression, because il does not refer to a real person or thing.

In il faut, the il is a dummy subject or impersonal il. It does not mean he.

French often requires a subject, even when English uses something more flexible:

• Il pleut. = It’s raining.
• Il faut partir. = It is necessary to leave.

So here, il is just part of the fixed expression il faut.

Because after il faut, French normally uses an infinitive when the action is general and no separate subject is introduced.

So:

• Il faut tailler ce rosier. = You need to prune this rosebush.

Structure:

• il faut + infinitive

Other examples:

• Il faut manger. = You have to eat.
• Il faut attendre. = You must wait.

If French wants to specify a different subject, it can use il faut que + subjunctive:

• Il faut que tu tailles ce rosier. = You must prune this rosebush.

Here tailler means to prune or to trim.

In a gardening context, tailler un rosier means cutting back the plant in order to help it grow better or flower better.

So although tailler can generally mean to cut or to shape, in this sentence prune is the best translation.

Un rosier is a rosebush or rose plant, not a single rose flower.

So:

• une rose = a rose (the flower)
• un rosier = a rosebush

That is why the sentence is about pruning it: you prune the plant, not the individual flower.

Because rosier is masculine singular and begins with a consonant sound.

French demonstratives are:

• ce
  • masculine singular noun beginning with a consonant
• cet
  • masculine singular noun beginning with a vowel or mute h
• cette
  • feminine singular noun
• ces
  • plural noun

So:

• ce rosier = this rosebush
• cet arbre = this tree
• cette fleur = this flower
• ces rosiers = these rosebushes

French often uses the definite article where English does not.

So:

• avant l’été = before summer
• literally: before the summer

This is very normal in French.

You will often see:

• en hiver = in winter
• au printemps = in spring
• avant l’été = before summer
• après l’automne = after autumn

The l’ is just le or la shortened before a vowel. Since été begins with a vowel sound, French writes l’été.

Because the sentence is expressing a purpose involving the rosebush as the subject of a new verb.

• pour
  • infinitive is used when the subject stays the same
• pour que
  • subjunctive is used when a new clause with its own subject follows

Here:

• pour qu’il fleurisse mieux = so that it flowers better

The subject of fleurisse is il, referring to ce rosier.

Compare:

• Je taille le rosier pour le garder en bonne santé.
  = I prune the rosebush to keep it healthy.
  Same subject for the infinitive idea.

• Je taille le rosier pour qu’il fleurisse mieux.
  = I prune the rosebush so that it flowers better.
  New clause: it flowers.

So pour qu’ is the correct structure here.

Because pour que is one of the expressions that normally triggers the subjunctive in French.

So:

• pour qu’il fleurisse
  not
• pour qu’il fleurit

The subjunctive is used here because the clause expresses a goal, desired result, or intention, not a simple statement of fact.

The verb is fleurir (to flower / to bloom), and the subjunctive form here is:

• qu’il fleurisse

Other examples:

• Je viens pour que tu comprennes. = I’m coming so that you understand.
• On arrose la plante pour qu’elle pousse. = We water the plant so that it grows.

Because rosier is the logical singular masculine noun that can flower.

So in:

Il faut tailler ce rosier avant l’été pour qu’il fleurisse mieux.

the il means the rosebush.

French often uses a pronoun like this instead of repeating the noun:

• ce rosier ... qu’il fleurisse
• literally: this rosebush ... so that it may flower

Mieux means better.

So:

• fleurir mieux = to flower better
• qu’il fleurisse mieux = so that it flowers better

In more natural English, you might also say:

• so that it blooms better
• so that it produces better flowers
• so that it flowers more successfully

Mieux is the comparative form related to bien:

• bien = well
• mieux = better

Example:

• Il chante bien. = He sings well.
• Il chante mieux. = He sings better.

Yes, fleurisse comes from fleurir.

The form fleurisse is the present subjunctive, while fleurit is usually the present indicative form.

Compare:

• Il fleurit au printemps. = It flowers in spring.
  Indicative: statement of fact

• pour qu’il fleurisse mieux = so that it flowers better
  Subjunctive: purpose/desired result

So the change in form is caused by the grammar after pour que.

Not in the same way.

Avant de + infinitive is used when the subject is the same:

• Il faut tailler ce rosier avant de partir.
  = You have to prune this rosebush before leaving.

But here the sentence means before summer, not before doing something, so French uses:

• avant l’été = before summer

So avant de would not fit this exact meaning.