There are different uses of the noun "reason" here.
B,C,D) For these it is a countable noun.
I have reasons for doing this.
Note the use of the plural. This looks very similar to (A) but is using "reason" as a countable noun.
0) Another use of "reason" is an uncountable noun.
There is reason in madness.
A) Example (A) is a phrase "to have reason". Here "reason" means that what you do is reasonable, you could justify it. It is from the uncountable noun. So what you do can be justified.
I have reason to believe you.
I have good reason to do this.
Or you could justify the actions of someone else.
He has reason to do that.
Sometimes, you can justify your expectation of something even if it is not true or provably true,
He has reason to know.
There's not actually a single answer to this... it depends on the company structure, which you haven't included in your question, but that's ok.
Here's your sentence:
My position was _____ e-shop back-end and front-end software developer.
If you were the only person at that company with that job title, the correct article is "the".
My position was the e-shop back-end and front-end software developer.
Now, if you were one of two or more people with this position, then you would use the indefinite article. Here, the choice is between "a" and "an". The rule is
- Use "a" when proceeding a consonant sound
- Use "an" when proceeding a vowel sound
In your case, e-shop is a vowel sound, so you'd go with "an".
My position was an e-shop back-end and front-end software developer.
You can use neither article, though, if you wish. Because this position is sort of your "title", you could certainly opt to exclude them.
My position was e-shop back-end and front-end software developer.
If it helps, you can imagine "e-shop back-end and front-end software developer" is in quotes but don't include the quotes on your resume/CV.
As an added note, in the versions with articles, you don't necessarily need the word "position":
I was the/an e-shop back-end and front-end software developer.
This version is perfectly fine and (to my ears) sounds slightly better.
Best Answer
"Few" and "a few" are significantly different in grammar and meaning.
"A few" is neutral: it simply refers to a small number of whatever, without suggesting any expectations.
"Few" triggers negative polarity expressions (such as "any" and "yet"), and emphasises the smallness of the number, and suggests that a larger number might have been expected.
Contrast this with:
both of which are emphasising that the number of people is small.
A few with negative polarity items is not grammatical:
There is no real answer to your question: like most "why" or "what is the reason" questions about language, the only answer is "because that's the way it is".