1. Letters to the Editor

2. Letters for the Editor
3. Letters which are sent to the Ediror.
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Can we use #2? Does #1 mean #3?

4. They will be useful to someone.
5. They will be useful for someone.
[Which preposition do we have to use? Are both okay?]

1. is usually used. 2. and 3. are not

Both prepositions in 4 and 5 are commonly used.

1. Letters to the Editor is the most commonly used phrase to refer to letters that are written and sent to the editor of a newspaper, magazine, or any other publication. It follows the typical pattern of addressing the recipient of the letter.

2. Letters for the Editor is less common, but it can also be used to refer to letters intended for the editor. While it is less commonly used, it conveys a similar meaning as "Letters to the Editor." The difference lies in the emphasis on the intended recipient rather than the act of sending the letter.

3. Letters which are sent to the Editor is a longer and more formal way of expressing the same idea. The phrase emphasizes the action of sending the letters and often indicates a passive voice construction.

Regarding the use of prepositions:

4. Both "useful to someone" and "useful for someone" are grammatically correct and can be used interchangeably in many contexts. However, there might be some subtle differences in meaning.

- "Useful to someone" suggests that something has a beneficial impact or value in relation to someone.
Example: The information in these letters may be useful to the editor.

- "Useful for someone" implies that something has a purpose or function that benefits someone.
Example: These letters will be useful for the editor in understanding public opinion.

In most cases, the choice between "to" and "for" depends on the specific context and the intended meaning. Both phrases are commonly used, and the decision often comes down to personal preference or stylistic choices.