Two words, one unit
A compound word combines two or more words into a unit with a new meaning. Tooth and brush give us toothbrush: a brush for teeth.
Cambridge’s guide to compounds shows why this is different from adding a prefix or suffix. The pieces can already have lives as words.
A space is not the deciding clue
Compounds can be written as one word, with a hyphen or with a space. Toothbrush, short-sighted and nail polish show the three possibilities.
The important question is how the words work together. Nail polish names one familiar product even though a space remains in the spelling.
Look at the whole meaning
A sunflower is a particular kind of plant, rather than any flower placed in sunshine. The combined meaning matters.
For spelling, use a current dictionary rather than assuming every compound follows the same pattern. To finish the course, follow a word backwards through time with etymology.