Дедушка раскатал тесто скалкой и сказал, что пирог будет тонким.

Because some masculine nouns in Russian, especially family words and informal words for people, end in -а or -я.

So дедушка is grammatically masculine, even though its ending looks like a typical feminine ending.

That is why the verbs are masculine too:

• дедушка раскатал
• дедушка сказал

Other similar words:

• папа = dad
• дядя = uncle
• юноша = young man

So the ending does not always tell you the gender by itself.

Раскатал is the past tense masculine singular form of раскатать, which means to roll out.

Here it describes a completed action:

• раскатал тесто = rolled out the dough

Why this exact form?

• past tense: the action happened in the past
• masculine singular: it agrees with дедушка
• perfective aspect: раскатать focuses on the action as completed

Compare:

• раскатывал = was rolling out / used to roll out
• раскатал = rolled out, finished rolling out

So раскатал is used because the sentence presents the action as a finished step.

Тесто is the direct object, so it is in the accusative case. But for many inanimate neuter nouns, the accusative looks exactly the same as the nominative.

So:

• nominative: тесто
• accusative: тесто

That is why the form does not visibly change.

This is very common in Russian. For example:

• я вижу окно = I see the window
  (окно also stays the same)

Because it means with a rolling pin, and Russian often uses the instrumental case to show the tool or means used to do something.

So:

• скалка = rolling pin
• скалкой = with a rolling pin

In English, we normally need with. In Russian, the instrumental ending often does that job by itself.

So:

• раскатал тесто скалкой = rolled out the dough with a rolling pin

Usually, no—not with the same meaning.

• скалкой = with a rolling pin as the instrument
• с скалкой usually means together with a rolling pin or having a rolling pin

For tools, Russian normally uses the instrumental without с:

• писать ручкой = to write with a pen
• резать ножом = to cut with a knife
• раскатать тесто скалкой = to roll out dough with a rolling pin

So скалкой is the natural choice here.

Because что introduces a subordinate clause:

• сказал, что пирог будет тонким
• said that the pie would be thin

In Russian, when a clause like что... is attached to a main clause, it is normally separated by a comma.

Main clause:

• Дедушка раскатал тесто скалкой и сказал

Subordinate clause:

• что пирог будет тонким

So the comma is required.

Here что means that and introduces the content of what was said.

So:

• сказал, что... = said that...

This is one of the most common uses of что in Russian.

Examples:

• Он сказал, что устал. = He said that he was tired.
• Она знает, что это правда. = She knows that it is true.

So in this sentence, что connects said with the statement that follows.

Будет is the future tense of быть (to be) for he/she/it.

So:

• пирог будет тонким = the pie will be thin

The grandfather is talking about the result in the future, after the dough has been rolled out and the pie is made or baked.

Why singular?

Because the subject of this clause is пирог, which is singular.

Compare:

• пирог будет = the pie will be
• пироги будут = the pies will be

Because after быть in the future or past, Russian often uses the instrumental case for a predicate noun or adjective, especially when describing what something will be like as a state or result.

So:

• пирог будет тонким = the pie will be thin

Here:

• тонкий = dictionary form
• тонким = instrumental masculine singular

This is a very common pattern:

• Он был усталым. = He was tired.
• Она будет счастливой. = She will be happy.
• Суп будет горячим. = The soup will be hot.

You may sometimes encounter nominative forms in related structures, but тонким is the normal and natural choice here.

Because the first part talks about the dough, but the second part talks about the finished food that will result from it.

• раскатал тесто = rolled out the dough
• пирог будет тонким = the pie will be thin

So the sentence moves from the preparation step to the expected result.

This is completely natural: you roll out dough, but the thing that ends up being thin is the pie.

Пирог is a broad Russian word for a baked dish made from dough with some filling. In English, the closest translation is often pie, but depending on context it can overlap with ideas like a savory pie, baked pastry, or even something more cake-like.

In this sentence, pie is a good translation because the context is:

• rolling out dough
• talking about how thin the final product will be

So пирог here is best understood as pie.

Because Russian does not have articles like English a/an/the.

So:

• дедушка can mean grandfather or the grandfather
• тесто can mean dough or the dough
• пирог can mean a pie or the pie

Context tells you which meaning is intended.

In this sentence, English naturally uses the in translation, but Russian simply does not need a separate word for that.

Yes, Russian word order is more flexible than English word order.

The sentence as given is very natural and neutral:

• Дедушка раскатал тесто скалкой и сказал, что пирог будет тонким.

But other orders are possible, depending on emphasis. For example:

• Скалкой дедушка раскатал тесто...
  This emphasizes the tool.
• Дедушка сказал, что пирог будет тонким, и раскатал тесто скалкой.
  This changes the sequence and focus.

Russian uses word order not only for grammar, but also for information focus, emphasis, and style. Still, the original version is a normal default order.

Because both verbs refer back to the same subject: дедушка.

In the Russian past tense, verbs agree with the subject in gender and number:

• masculine singular: -л
• feminine singular: -ла
• neuter singular: -ло
• plural: -ли

So here:

• раскатал
• сказал

If the subject were feminine, you would get:

• Бабушка раскатала тесто скалкой и сказала...

So the verb endings help show who is doing the action.