1, He has put all his money into buying that house.

2. He put all the apples into the fridge.

3. She put all the dolls into the box.
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Are they all grammatical? Can we use 'in' instead of 'into'?

1 must have 'into,' but 2 and 3 can have either word.

Yes, all the sentences are grammatically correct. However, using "in" instead of "into" in these sentences would change the meaning slightly.

Let's analyze each sentence:

1. "He has put all his money into buying that house."
- Here, "into" is used to show the direction of the money. It implies that he invested his money with the purpose of purchasing the house.

2. "He put all the apples into the fridge."
- Again, "into" is used to indicate movement or direction. It implies that the apples were placed inside the fridge intentionally.

3. "She put all the dolls into the box."
- Here, "into" is used to express that the dolls were placed inside the box deliberately.

Using "in" instead of "into" would still make these sentences grammatically correct, but it would convey a different meaning:

1. "He has put all his money in buying that house."
- This sentence suggests that he used his money for the purpose of buying the house, but it doesn't necessarily mean that the money is inside the house.

2. "He put all the apples in the fridge."
- This sentence implies that the apples are already inside the fridge; there is no indication of intentional movement.

3. "She put all the dolls in the box."
- This sentence indicates that the dolls are already inside the box; there is no focus on the action of placing them there intentionally.

So, while both "into" and "in" can be used in these sentences, they slightly alter the meaning depending on the context.