Si no la riegas esta tarde, la rosa puede marchitarse antes del fin de semana.

Because si no here literally means if not:

• si = if
• no = not

So Si no la riegas... means If you don’t water it...

Sino is a different word. It usually means but rather or except, after a negative.

• No es roja, sino rosa. = It isn’t red, but rather pink.

So in this sentence, si no is the only correct choice.

That la is a direct object pronoun meaning it.

It refers to la rosa. Since rosa is a feminine noun in Spanish, the pronoun is la.

So:

• la riegas = you water it

In English we would say it, but Spanish uses lo/la depending on grammatical gender.

Spanish often leaves out subject pronouns because the verb ending already shows who the subject is.

• riegas = you water
• the -as ending tells you it is tú

So Si no la riegas... already means If you don’t water it...

You could say Si tú no la riegas..., but that would add emphasis, as if contrasting you with someone else.

Because this is a real or possible condition, and after si Spanish normally uses the indicative, not the subjunctive.

So:

• Si no la riegas... = correct
• Si no la riegues... = incorrect

A good pattern to remember is:

• Si estudias, apruebas. = If you study, you pass.
• Si llueve, nos quedamos en casa. = If it rains, we stay home.

The subjunctive is not normally used right after si in this kind of sentence.

Regar is a stem-changing verb. Its e changes to ie in stressed forms.

So:

• regar
• riego
• riegas
• riega
• riegan

But:

• regamos
• regáis

This is the same kind of change you see in verbs like:

• pensar → piensas
• cerrar → cierras

So riegas is just the normal tú form of regar.

Esta tarde means this afternoon or this evening, depending on context.

In Spain, la tarde usually covers the afternoon and can extend into the early evening. So the exact English translation depends on when the speaker means.

In this sentence, it simply means later today, during the afternoon/evening period.

Because esta here is a demonstrative adjective modifying tarde:

• esta tarde = this afternoon/evening

Modern Spanish spelling does not normally put accents on este, esta, ese, esa, etc.

So esta tarde is the standard spelling.

Marchitarse means to wilt or to wither.

The se is part of the verb as it is commonly used when something becomes wilted.

• La rosa puede marchitarse. = The rose may wilt.

Compare:

• marchitar algo = to make something wilt
• marchitarse = to wilt

So here the rose is not doing an action to itself in a literal reflexive sense; marchitarse is just the normal verb form for becoming wilted.

Yes. Both are correct.

With a conjugated verb plus an infinitive, pronouns can usually go:

• before the conjugated verb, or
• attached to the infinitive

So both of these work:

• puede marchitarse
• se puede marchitar

They mean the same thing.

Here puede expresses possibility, not ability.

So in English it could be:

• can
• may
• might
• could

Depending on style, The rose may/might wilt is probably the clearest idea.

So puede here is closer to it may happen than to it is able to.

Because puede marchitarse sounds less definite. It expresses a possible consequence:

• puede marchitarse = it may/could wilt

If you said:

• se marchitará

that would sound more certain:

• it will wilt

So the speaker is giving a warning about what may happen, not stating it as guaranteed.

Because del is the contraction of de + el.

• antes de + el fin de semana → antes del fin de semana

This contraction is normal in Spanish.

So:

• del = de + el
• al = a + el

As a fixed expression, el fin de semana means the weekend.

So:

• antes del fin de semana = before the weekend

Even though the words literally look like end of week, the normal meaning is weekend.

If you wanted to say before the week ends, you would usually say something like:

• antes de que termine la semana