A rega, a umidade e a drenagem são importantes.

Because a is the feminine singular definite article, equivalent to the.

All three nouns here are feminine singular nouns:

• a rega
• a umidade
• a drenagem

In Portuguese, it is very normal to repeat the article before each item in a list. So a rega, a umidade e a drenagem sounds natural and complete.

Yes, grammatically it is the definite article the, but Portuguese often uses the definite article in general statements where English does not.

So Portuguese says:

A rega, a umidade e a drenagem são importantes.

But natural English usually drops the article:

Watering, humidity, and drainage are important.

This is a very common difference between English and Portuguese. Portuguese often uses o/a/os/as with general concepts more often than English does.

Because the subject is plural.

The subject is made of three things joined by e:

• a rega
• a umidade
• a drenagem

When two or more nouns are joined this way, Portuguese normally uses a plural verb. Since the verb is ser, the plural form is são.

• singular: A drenagem é importante.
• plural: A rega, a umidade e a drenagem são importantes.

Because the adjective has to agree in number with the subject.

Since the subject is plural, the adjective is plural too:

• singular: importante
• plural: importantes

A useful detail: importante does not change for masculine vs. feminine in the singular. It only changes for number here:

• o solo importante — not natural semantically, but grammatically masculine singular
• a rega importante — feminine singular
• os fatores importantes — masculine plural
• as condições importantes — feminine plural

So in this sentence, importantes is plural because there are three items.

Yes. That is why they all take a.

• a rega
• a umidade
• a drenagem

For drenagem, this is especially useful to memorize because many nouns ending in -agem are feminine in Portuguese:

• a viagem
• a mensagem
• a drenagem

Yes, rega is a normal word, especially in gardening and plant care. It means watering.

Irrigação is also possible, but it is usually more technical or more associated with agriculture, irrigation systems, or larger-scale watering.

So:

• rega = watering, especially everyday plant care
• irrigação = irrigation, often more technical or large-scale

In a sentence about houseplants or gardening advice, rega sounds very natural.

In Brazilian Portuguese, umidade is the standard form.

In European Portuguese, you will often see humidade.

So if you are learning Brazilian Portuguese, umidade is the form you should use.

Sometimes Portuguese does omit articles in compact styles, but for a learner, the safest and most natural pattern is to keep them:

A rega, a umidade e a drenagem são importantes.

That sounds fully standard.

You may occasionally see reduced versions in notes, headings, or more compressed writing, but repeating the article before each noun is the clearest default.

Mostly yes. This is just a list of three items.

One small difference is that Portuguese normally does not use a comma before e in a simple list. So this sentence is written naturally as:

A rega, a umidade e a drenagem são importantes.

That is the standard punctuation.

It naturally suggests both ideas at once:

• the three factors as a group are important
• each listed factor is an important element

That is how list subjects usually work in Portuguese, just as in English. The sentence presents rega, umidade, and drenagem as key factors.

A rough Brazilian Portuguese pronunciation would be:

ah HEH-gah, ah oo-mee-DAH-jee ee ah dreh-NAH-zhem SAH-oo een-por-TAHN-chees

A few useful pronunciation notes:

• r in rega is often pronounced like an English h sound at the beginning of a word in many Brazilian accents.
• d before i in umidade often sounds like j in jeep in Brazilian Portuguese.
• g in drenagem sounds like the zh sound in measure.
• são is nasal, so it does not sound exactly like sow in English.

If you want, you can think of the rhythm as:

a RE-ga, a umi-DA-de e a drena-GEM são impor-TAN-tes

with stress on RE, DA, GEM, são, and TAN.