Si no reguem el jardí avui, demà la terra serà massa seca per plantar.

Because Catalan normally uses the present subjunctive after si in this kind of negative or uncertain future condition.

So:

• si no reguem = if we don’t water
• serà = it will be

This is a very common pattern in Catalan:

• Si plou, ens quedarem a casa. = If it rains, we’ll stay home.
• Si no véns, començarem sense tu. = If you don’t come, we’ll start without you.

For English speakers, it can feel unusual because English uses a present form too, but Catalan often uses a subjunctive-looking form in these cases.

Reguem is the 1st person plural form of regar (to water), meaning we water or we water/are to water depending on context.

Here it means:

• reguem = we water

The subject we is not stated separately because Catalan verb endings usually already show the subject.

Compare:

• rego = I water
• regues = you water
• rega = he/she waters
• reguem = we water
• regueu = you all water
• reguen = they water

Catalan often drops subject pronouns when they are not needed, because the verb ending already tells you who the subject is.

So:

• reguem already means we water
• nosaltres reguem is possible, but only if you want emphasis or contrast

For example:

• Si no reguem el jardí... = neutral
• Si nosaltres no reguem el jardí... = more emphatic, like if we don’t water the garden...

This is similar to Spanish and unlike normal English, where the subject pronoun is usually required.

Catalan uses definite articles very naturally, often more than English does.

So:

• el jardí = the garden
• la terra = the soil / the ground

In this sentence, the speaker is talking about a specific garden and its soil, so the articles sound completely natural.

English sometimes uses no article where Catalan prefers one, but here both languages use one.

Here la terra means the soil / the ground, not the planet Earth.

Terra can mean different things depending on context:

• la Terra = the Earth (planet)
• la terra del jardí = the soil in the garden
• terra can also mean land

In this sentence, because we are talking about watering a garden and planting, la terra clearly means soil.

Serà is the future tense of ser (to be), 3rd person singular:

• serà = it will be

The subject is la terra, which is singular, so Catalan uses the singular future form:

• demà la terra serà massa seca = tomorrow the soil will be too dry

A quick comparison:

• sóc = I am
• és = it is / he is / she is
• serà = it will be

Because seca agrees with terra, which is a feminine singular noun.

In Catalan, adjectives usually agree in gender and number with the noun they describe:

• terra = feminine singular
• seca = feminine singular form of sec (dry)

Compare:

• el sòl sec = the dry ground (masculine singular)
• la terra seca = the dry soil (feminine singular)
• les plantes seques = the dry plants (feminine plural)

Massa means too in the sense of excessively.

So:

• massa seca = too dry

In Catalan, massa usually goes before the adjective:

• massa car = too expensive
• massa tard = too late
• massa seca = too dry

Do not confuse it with:

• molt = very / much
• massa = too much / too

So:

• molt seca = very dry
• massa seca = too dry

Per plantar means to plant or more literally for planting.

In this sentence:

• massa seca per plantar = too dry to plant

Catalan often uses per + infinitive to express purpose or suitability:

• aigua per beure = water to drink
• paper per escriure = paper to write on / for writing
• massa seca per plantar = too dry to plant

You may also sometimes see per a in other contexts, but per plantar is perfectly natural here.

Yes. This sentence is a very typical real future condition:

• Si + present/subjunctive-type form, then
• future

So the structure is:

• Si no reguem el jardí avui, demà la terra serà massa seca per plantar.

This is similar in meaning to English:

• If we don’t water the garden today, tomorrow the soil will be too dry to plant.

It expresses a realistic consequence, not a hypothetical past or an unreal situation.

Yes, Catalan word order is fairly flexible, and that version is also possible:

• Si no reguem el jardí avui...
• Si no reguem avui el jardí...

Both are grammatical. The difference is mostly one of emphasis or rhythm.

• el jardí avui keeps the object close to the verb
• avui el jardí puts a little more focus on today

The original sentence sounds very natural and neutral.

The comma separates the if-clause from the main clause:

• Si no reguem el jardí avui, = dependent clause
• demà la terra serà massa seca per plantar. = main clause

This is also common in English:

• If we don’t water the garden today, tomorrow the soil will be too dry to plant.

It helps the sentence read more clearly.

A helpful approximate pronunciation is:

• reguem ≈ reh-GHEM

A few points:

• the g before u/e keeps a hard g sound because of the u
• the ending -em is not pronounced like English them
• stress falls on the last syllable here: re-GUEM

Also:

• jardí is stressed on the last syllable because of the accent: jar-DÍ
• serà is also stressed on the last syllable: se-RÀ

Yes, depending on nuance.

Possible alternatives:

• el sòl = the ground / soil
• la terra = the soil / earth

In a gardening context, la terra is very natural because it suggests the earth/soil you plant in.
El sòl can sound a bit more technical or general.

So the original sentence is natural and idiomatic for everyday speech.