1. It's a very simple recipe.

2. Spaghetti is a very simple recipe.
3. To make speghetti is a very simple recipe.
4. How to make spghetti is a very simple recipe.
5. ______ is a very simple recipe.
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Which one is grammatical? Which terms can be put in the blank in #5?
Is 'It' in #1 a "situation it" which doesn't refer to a specific one? Or can 'it' refer to 'spaghetti' or other things?

They're all grammatical. "It" refers to a specific recipe and would be understood to mean something, like spaghetti, already mentioned earlier.

Among the options provided, sentences number 1, 2, and 5 are grammatical and convey similar meanings.

Sentence number 1, "It's a very simple recipe," uses the pronoun "it" as a placeholder for an unspecified subject. In this case, "it" could refer to any simple recipe, not necessarily spaghetti.

Sentence number 2, "Spaghetti is a very simple recipe," directly states that spaghetti is a simple recipe.

Sentence number 5 provides an open-ended statement with a blank, leaving room for various possibilities to complete the sentence. Some examples of terms that could be placed in the blank are "Pancakes," "Omelette," or "Stir-fry." Essentially, any dish or recipe that is simple can be inserted.

Regarding the other sentences:

Sentence number 3, "To make spaghetti is a very simple recipe," is not grammatically correct. It appears to be a slightly inaccurate attempt at expressing the simplicity of making spaghetti, but the structure is not correct.

Sentence number 4, "How to make spaghetti is a very simple recipe," is also not grammatically correct. It seems like an incomplete phrase rather than a proper sentence.

In summary, sentence #1 uses a generic pronoun, while sentence #2 explicitly mentions spaghetti as the simple recipe. In sentence #5, various dish or recipe names can be inserted to complete the statement.