1. His hardest decision was not to go abroad.

2. His hardest decision wasn't to go abroad.

3. His hardest decision was / not to go abroad.

4. His hardest decision was to not go abroad.

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#1 is a little difficult to comprehend. Sentence 1 has two meanings, right? I mean #2 and #3. Is Sentence 4 grammatical? Would you explain the differences among the sentences? Which expressions do we have to use? Thank you.

1. His hardest decision was not to go abroad - This sentence means that the person's most difficult decision was the choice to not go abroad. It implies that going abroad was an option and the decision to not go was difficult for the person.

2. His hardest decision wasn't to go abroad - This sentence means that going abroad was not the person's most challenging decision. It suggests that the person made other decisions that were more difficult than the decision to go abroad.

3. His hardest decision was / not to go abroad - This sentence seems to be incomplete or missing a word. It is not clear what "was / not" means in this context. It could be a typo or a mistake in writing.

4. His hardest decision was to not go abroad - This sentence is grammatically correct. It means that the person's most challenging decision was the choice to not go abroad. It is similar to sentence 1 in meaning.

In terms of expressing the idea that not going abroad was a challenging decision, either sentence 1 or sentence 4 would be appropriate. Both sentences convey the same meaning, but sentence 4 is slightly more commonly used in formal writing. However, sentence 2 suggests that the person had other harder decisions and sentence 3 lacks clarity.

All of the sentences you provided are grammatically correct, but they convey slightly different meanings.

1. "His hardest decision was not to go abroad."
This sentence means that not going abroad was the most difficult decision for him. It suggests that going abroad was something he wanted or needed to do, but he ultimately decided against it.

2. "His hardest decision wasn't to go abroad."
This sentence means that going abroad was not the most difficult decision for him. It implies that he made other decisions that were more challenging or had a greater impact on him.

3. "His hardest decision was / not to go abroad."
This sentence is a combination of the first two sentences. The slash (/) indicates that either "was" or "was not" can be used to complete the sentence. If "was" is used, it conveys the same meaning as sentence 1. If "was not" is used, it conveys the same meaning as sentence 2.

4. "His hardest decision was to not go abroad."
This sentence is grammatically correct, but it has a slightly different structure. Instead of using "not to," it uses "to not." This changes the emphasis of the sentence to the decision itself rather than the act of not going abroad. It suggests that going abroad was an option, but he ultimately decided against it, which was the most difficult decision for him.

The differences among the sentences lie in the emphasis and interpretation of the decision. The expressions "not to" and "to not" are often interchangeable, but they can slightly shift the focus of the sentence depending on their placement.

Let's break down the differences among the sentences you provided:

1. "His hardest decision was not to go abroad." - This sentence means that the most challenging decision he had to make was the decision of NOT going abroad.

2. "His hardest decision wasn't to go abroad." - This sentence means that the most difficult decision he had to make was something other than choosing to go abroad. The word "wasn't" is a contraction of "was not."

3. "His hardest decision was / not to go abroad." - I assume you meant "was or was not to go abroad." This sentence implies that the hardest decision he had to make could be either choosing to go abroad or choosing not to go abroad.

4. "His hardest decision was to not go abroad." - This sentence means that the most challenging decision he had to make was the decision of choosing to not go abroad.

Grammatically, all the sentences are correct, but they convey slightly different meanings.

To clarify, if you want to express that the decision to not go abroad was the most difficult, you can use either sentence 1 or sentence 4.

If you want to convey that the most difficult decision was something other than choosing to go abroad, you can use sentence 2.

Sentence 3 is not standard usage and may cause confusion.