We all know that Greek does not have a specific word for “a,” the indefinite article. But Greek does have several ways to approximate the same meaning. For example, εἷς normally means “one,” but it can mean “one” in the sense of “someone.” An articular participle can also have an indefinite thrust, as in John 3:16 (ὁ πιστεύων).
But normally we use “a” when translating an anarthrous Greek noun, but even then there can be ambiguity. Take John 3:25 for example. “Now a discussion arose between some of John’s disciples and a Jew (μετὰ Ἰουδαίου) over purification” (ESV, see also NASB, CSB, NRSV; the KJV follows a plural variant reading, “the Jews”).
So why do several translation says, “a certain Jew” (NIV, NET, NLT)? Because in today’s polite speech we don’t say “a Jew,” just like we don’t say “a ____” about any ethnic group. It feels pejorative to say this. This is not being politically correct; it is being polite. So “certain” softens the affect of the words.