Дедушка сказал, что в теплице будет теплее, и поэтому помидоры дадут хороший урожай раньше.

Сказал is the perfective past tense of сказать and means said as a completed act: Grandpa said...

Russian often uses сказать when referring to one finished statement.
By contrast, говорил from говорить is imperfective and often means something like:

• was saying
• used to say
• spoke
• told in a more ongoing or repeated sense

So in this sentence, Дедушка сказал is the natural choice because it refers to one completed remark.

Что means that and introduces reported speech or a subordinate clause:

• Дедушка сказал, что... = Grandpa said that...

This is extremely common in Russian after verbs like:

• сказать = to say
• знать = to know
• думать = to think
• понимать = to understand

English often drops that, but Russian usually keeps что in this structure.

Because the preposition в here means in, and after в with location, Russian uses the prepositional case.

• теплица = greenhouse
• в теплице = in the greenhouse

Case breakdown:

• nominative: теплица
• prepositional: в теплице

This is a standard pattern:

• в доме = in the house
• в школе = at school / in the school
• в саду = in the garden

Because the sentence refers to the future.

• теплее = warmer
• будет теплее = it will be warmer

Russian often uses быть + comparative adjective to talk about future states:

• будет лучше = it will be better
• будет легче = it will be easier
• будет холоднее = it will be colder

So в теплице будет теплее means it will be warmer in the greenhouse.

Теплее is the comparative form of тёплый / тепло and means warmer.

Russian comparatives often end in:

• -ее
• -ей
• sometimes other patterns

Examples:

• тёплый → теплее = warm → warmer
• холодный → холоднее = cold → colder
• быстро → быстрее = quickly / fast → faster

A useful point: Russian comparatives do not always need an explicit than phrase.
So в теплице будет теплее can mean it will be warmer in the greenhouse, with the comparison understood from context.

Поэтому means therefore, so, or that’s why.

In this sentence:

• ..., и поэтому помидоры дадут...
• ..., and therefore the tomatoes will produce...

The combination и поэтому is very common and natural. It links the two ideas:

1. It will be warmer in the greenhouse.
2. As a result, the tomatoes will yield earlier.

So поэтому marks the consequence.

Because помидоры is the subject of the verb дадут.

• помидор = tomato
• помидоры = tomatoes

The verb agrees with it in plural:

• помидоры дадут = the tomatoes will give / yield

Even though in English we might say the tomato plants rather than the tomatoes, Russian commonly says помидоры when talking about the crop.

Yes, дать usually means to give, but in Russian it is also used in expressions about plants and farming:

• дать урожай = to produce a crop / to yield

So here:

• помидоры дадут хороший урожай
  means
• the tomatoes will produce a good harvest / yield a good crop

This is a normal collocation in Russian.

Also, дадут is the future tense of the perfective verb дать:

• я дам
• ты дашь
• он/она даст
• мы дадим
• вы дадите
• они дадут

Because помидоры is plural, the sentence uses дадут.

Урожай means harvest or crop, and here it is the direct object of дадут.

So Russian uses the accusative case.
However, урожай is masculine singular inanimate, and for such nouns the accusative looks the same as the nominative:

• nominative: хороший урожай
• accusative: хороший урожай

That is why the form does not visibly change.

Раньше means earlier.

Like теплее, it is a comparative form, and Russian often uses comparatives without explicitly saying what they are compared to when the comparison is understood from context.

So here раньше means something like:

• earlier
• earlier than they otherwise would
• sooner

The sentence does not need a separate word for than because the comparison is implicit.

Russian word order is flexible, and adverbs like раньше are often placed near the end for natural flow or emphasis.

This sentence ends with раньше because that final word highlights the outcome:

• the tomatoes will yield a good harvest earlier

You could move words around in Russian, but the given order sounds natural and clear.

Yes. Russian word order is much more flexible than English word order, because case endings show grammatical roles.

For example, variants like these are possible:

• Дедушка сказал, что в теплице будет теплее, и поэтому раньше помидоры дадут хороший урожай.
• Дедушка сказал, что в теплице будет теплее, и помидоры поэтому дадут хороший урожай раньше.

But not all versions sound equally natural.
The original sentence is a good neutral version.

In general, Russian word order often changes to show:

• emphasis
• topic vs. new information
• style or rhythm

Yes. The commas are there for normal Russian clause structure.

• Дедушка сказал, что...
  A comma is needed before что because it introduces a subordinate clause.

• ..., и поэтому помидоры дадут...
  The comma before и поэтому separates the first clause from the second coordinated clause.

So the punctuation reflects the sentence’s structure:

1. Дедушка сказал
2. что в теплице будет теплее
3. и поэтому помидоры дадут хороший урожай раньше

Russian punctuation is often more rule-based than English punctuation in this kind of sentence.