1. We can invite all our friends if we have a bigger house.

2. We will invite all our friends if we have a bigger house.

3. We could invite all our friends if we had a bigger house.

4. We would invite all our friends if we had a bigger house.

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Are the four sentences grammatical? Would you let me know the diferences? Thank you.

#2 is factual. The others are possibilities. All are grammatically correct.

Yes, all four sentences are grammatically correct. However, there are some differences in meaning and usage between them:

1. "We can invite all our friends if we have a bigger house."
This sentence suggests that if the condition of having a bigger house is fulfilled, it is possible or permissible for us to invite all our friends. It implies a more realistic probability of obtaining a bigger house in the future.

2. "We will invite all our friends if we have a bigger house."
This sentence expresses a stronger commitment or intention to invite all our friends if we obtain a bigger house. It implies a higher degree of certainty or determination compared to sentence 1.

3. "We could invite all our friends if we had a bigger house."
This sentence introduces an unreal or hypothetical condition. It implies that in the current situation, we don't have a bigger house, but if we did, we would be able to invite all our friends. It suggests a limited possibility or feasibility.

4. "We would invite all our friends if we had a bigger house."
Similar to sentence 3, this sentence also presents a hypothetical situation. It suggests that if the condition of having a bigger house were met, we would definitely invite all our friends. It implies a stronger certainty or inclination compared to sentence 3.

Overall, the differences lie in the level of probability, commitment, and hypothetical nature conveyed by each sentence.