"Mi abuelo es un carpintero."
Translation:My grandfather is a carpenter.
This question fills many pages of Duo discussion forum every time a profession is mentioned. Do we use articles with professions or not. There seems to be much disagreement on this. Before Duo I had always learned NOT to use articles unless the profession were modified. Some people say to always use them. Duo is inconsistent. So I have come to the conclusion that I am not going to worry about it. In the rare occasion where I would have to use this, I am sure a native speaker would understand me whether or not I use an article. In the meantime, I continue to research other sources. We should not take Duo as the final authority.
From what I've figured out [Please someone correct me if I'm wrong], the indefinite article is used only before occupations and not professions. (It is actually also used for professions when it is modified - i.e. He is a good doctor)
As a native English speaker I was not aware of the difference between professions and occupations but apparently the difference is that professions require some form of higher qualification/learning (university).
So doctor, engineer, scientist etc. are professions and carpenter, builder, driver, delivery man etc. are occupations.
This link tries to explain the differences between occupations and professions
Although I don't agree with it 100%, especially the part about occupations not requiring deep knowledge, you can get the general idea of the differences.
The reason articles are not used with professions is that some professions used to or still do define a person's identity. Doctors, attorneys, priests, teachers. A carpenter is not what we call a professional, but a skilled person. The difference is historical. Duolingo just walks you through examples and lets you abstract the feel of what's right. A good grammar guide will likely help.