I saw two versions of a sentence "He is a teacher":
他是老师。 VS 他是一个老师。
After googling I've become even more confused by versions:
他是一位老师。 AND 他是一名老师。
Are they actually different? What is the difference? Is it possible to use these ‘一什么什么’ in questions?
Found one more version:
Which is said to be informal at this link.