Если завтра будет тепло, мы снова поедем на дачу и возьмём с собой лопату, лейку и семена.

Because тепло here is not an adjective describing a noun; it is a predicative adverb meaning warm in the sense of the weather will be warm.

• будет тепло = it will be warm
• тёплый is an adjective and must describe a noun:
  • тёплый день = a warm day
  • тёплая погода = warm weather

So in weather expressions, Russian often uses words like:

• холодно = cold
• тепло = warm
• жарко = hot

Example:

• Завтра будет холодно. = It will be cold tomorrow.

Yes. In Russian, after если (if), you normally use the tense that is natural for the situation, and for a future condition Russian often uses the future, unlike English.

So:

• Если завтра будет тепло...
  literally: If tomorrow it will be warm... natural English: If it’s warm tomorrow...

English usually uses present tense after if for future meaning, but Russian does not have to do that. Using будет here is completely normal and standard.

Because both actions are being presented as future results of the condition.

• если завтра будет тепло = if it is warm tomorrow
• мы снова поедем... = we will go again...
• и возьмём... = and we will take...

Russian is simply stating what will happen if the condition is met.

Also, both verbs are perfective, so their simple future forms mean a completed or single future action:

• поехать → поедем = we will go / set off
• взять → возьмём = we will take

Because поехать means to go by transport or to set off somewhere by vehicle, while пойти usually means to go on foot or to start going.

Going на дачу often implies traveling by car, bus, train, etc., so поедем is the natural choice.

Compare:

• Мы пойдём в магазин. = We’ll walk to the store.
• Мы поедем на дачу. = We’ll go to the dacha, likely by some form of transport.

На дачу means to the dacha / to the country house.

Russian uses на with some destinations where English uses to. Дача is one of those common nouns that takes на:

• ехать на дачу = to go to the dacha
• быть на даче = to be at the dacha

This is just the standard preposition used with this word. It is something learners usually need to memorize.

Compare:

• в город = to the city
• в дом = into the house
• на дачу = to the dacha

Снова means again.

So:

• мы снова поедем на дачу = we’ll go to the dacha again

It suggests this is not the first time; they have gone before and will repeat it.

A very close synonym is опять. In many contexts, both can mean again, though опять can sometimes sound a bit more like once again or even yet again, depending on tone.

С собой means with us / along with us.

So:

• возьмём с собой лопату... = we’ll take a shovel with us

This is a very common Russian expression:

• взять с собой = to take with you
• унести с собой = to carry away with you
• принести с собой = to bring with you

Because the verb is возьмём (we will take), с собой here specifically means with us.

Compare:

• Возьми с собой воду. = Take water with you.
• Я взял с собой книгу. = I took a book with me.

They are in the accusative case because they are the direct objects of возьмём (we will take).

Base forms:

• лопата = shovel
• лейка = watering can

Accusative singular:

• лопату
• лейку

This is the normal pattern for many feminine nouns ending in -а:

• книга → книгу
• машина → машину
• лопата → лопату
• лейка → лейку

So the sentence is literally saying we will take a shovel, a watering can, and seeds.

Because семена is the correct plural form of семя (seed).

This noun is irregular compared with more common noun patterns:

• singular: семя
• plural: семена

In this sentence, семена is also the accusative plural form, which for this noun looks the same as the nominative plural.

So:

• семена = seeds

This is just a noun form you need to learn as part of the word.

Because Если завтра будет тепло is a subordinate if-clause, and Russian normally separates such clauses with a comma.

So the structure is:

• Если завтра будет тепло, = If it’s warm tomorrow,
• мы снова поедем... = we’ll go again...

This comma is required in standard written Russian.

Yes. Russian word order is more flexible than English, though the original sentence is the most neutral and natural.

For example, you could also say:

• Если завтра будет тепло, снова поедем на дачу и возьмём с собой лопату, лейку и семена.
• Мы, если завтра будет тепло, снова поедем на дачу...

But changing the order can shift emphasis or sound more marked. The original version is straightforward and neutral:

• condition first
• then subject
• then actions
• then the list of things

That makes it a very natural model sentence for learners.

Because Russian often does not use a dummy subject like English it in weather and impersonal expressions.

English says:

• It will be warm
• It is cold
• It is dark

Russian simply says:

• Будет тепло
• Холодно
• Темно

There is no need for a separate word meaning it here. This is a normal feature of Russian grammar.