1. This digital camera is so small that it can fit in my pocket.

2. This digital camera is so small that it can fit my pocket.
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Which one is correct? Do we have to use 'fit in' or 'fit'? In Sentence 1, what is the part of speech of 'in' as in 'fit in'?

1 is more common.

have children write a row of each letter and then write words ( fin fit)

Both sentences are correct, but they have slightly different meanings.

In Sentence 1, "fit in" is used to indicate that the digital camera can be accommodated or contained within the pocket. It implies that the camera can physically occupy the space inside the pocket.

In Sentence 2, "fit" is used without the preposition "in," indicating that the camera can be placed or inserted into the pocket. It suggests that the camera can physically be placed inside the pocket, but doesn't necessarily indicate whether it takes up the entire space.

The part of speech of "in" in Sentence 1 is a preposition. Prepositions are words that show a relationship between a noun (or pronoun) and another word in the sentence. In this case, "in" is used to show the relationship between the verb "fit" and the noun "pocket." It clarifies where the camera is fitting.