Hi, guys

Could you run your eye over this sentence and tell me if it sounds o.k. to a native speaker, please?

1.a. Please, stick to alphanumeric characters when filling in this field and in doing so do not exceed a 300 symbols limit.

1.b. Please, stick to alphanumeric characters when filling in this field and in doing so do not exceed the 300 symbols limit.

I'm also in two minds over whether I should use "a" or "the". Could you advise me on which article fits the sentence well ?

Thanks in advance !
I'm not a native but here're my thoughts

If it's just an isolated field then I would use "a" but if it's part of a broader inquiry and they specifically want you to adhere to the 300 symbols limit used throughout the inquiry then use "the".
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I think "the" is appropriate in this case .

But I wonder what you mean by "and in doing" ?
I'm still none the wiser ))

Could a native speaker check my sentences, please?

Thanks again.
First, in both 1a and 1b, the usage of the word "symbols" is incorrect - in this case, the word "symbol" is actually plural in its own right. It's a very easy mistake as there's a vague, muddy (or, "unclear") difference between the usages. Keep reading - I've got examples of correct usage below Emotion: smile

Now, to answer your stated question:
1b is the better-written of the two

The article "a" in 1a is too passive - there is a 300 symbol limit. You're trying to demand something from the user (in this case, to avoid using more than 300 characters), but as you have it written, 1a doesn't impose a clear instruction upon the reader. You'd probably best use the article "a" if writing the statement as such:
"This field has a 300 symbol limit."

You're providing information in that case, but not declaring a demand.

Now, in 1b, using the article "the" expresses plainly that the limit is 300 symbols. You're demanding the user not exceed the limit.
"Do not exceed the 300 character limit."

It conveys a more forceful message, which is a good thing.

Students: We have free audio pronunciation exercises.