I need to know if it's OK to use have and some in this sentence, and why?
If I have some questions I will write you.
I've been told it is more typical to use any here instead of some. Is there a logical explanation for it?
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If I have some questions, I will write you is indeed less idiomatic than If I have any questions ... It's quite difficult to explain the difference formally (and of course, most native speakers would know this intuitively--in other words, they can use them correctly without much thought).
So, instead of trying to explain the usage of any and some systematically or formally, I will risk answering this question concisely, which could be more useful, by giving these paraphrases:
If I have some questions ... = If I have some amount/number of questions ...
If I have any questions ... = If I have any possible questions ...
And if you think about it, you may see that If I have some amount/number of questions, I will write you doesn't really work.
Saying "I will write you" makes perfect sense, by the way, so don't listen to the comment that says otherwise. People say it all the time. In addition, saying "some questions" just doesn't sound right. The only explanation is that "any questions" is a stock phrase in English. After a teacher explains a lesson, they ask, "Any questions?" not "Some questions?"
If I have some questions, I will write you.
First of all, "I will write you" doesn't make sense, and it should be "I will write to you". Secondly, the first part "have some", it's more proper to say "any", but however there is no problem in saying "have some" but using any is much better. It also implies that "some" would mean that if you had more then 3 questions, because "any" would state, if you have one or more, and "some" would state if you have more then 2 questions. "Any, A few, and Some" Any, would imply from 1, a few would imply from 2 - 3, and some 3 - 5. You see, it's about how many you have. It is also a stock phrase. No one would say "some" instead of "any". Additionally, the comma should connect the two sentences to create a compound sentence. The full sentence should be, if you are using British English:
If I have any questions, I will write to you.
Otherwise, just changing the some to any will be fine in US English, but it's more recommended to say the above statement because it is much more formal in both versions.