"El comandante escribe un libro."
Translation:The commander writes a book.
In the Spanish Army, “comandante” is the rank equivalent to an American or British major, or a French “commandant”. I do not know about all other Spanish-speaking countries, but at least a few use “mayor” for the same rank.
“Comandante” is also sometimes used in a more general way that does not specify a particular military rank, simply someone that is in command of something, even a civilian. For example, the pilot in command of an aeroplane is often called “comandante”.
"Commandant" is a good translation of "comandante" see this: http://www.spanishdict.com/translate/comandante
"Commandant" is ultimately from the Latin verb "mandare" which means "to order, command" http://latin-dictionary.net/definition/26326/mando-mandare-mandavi-mandatus
According to the RAE, "comandante" is from the Italian ":comandare" http://dle.rae.es/?id=9srIgi2 (Italian, of course, comes from Latin.)
"Commandant" would be a cognate of "comandante" . Generally, the cognate of a word is the best translation from Spanish. (Yes, "commandant" comes from the French (from about 1680); but the French comes from the Latin. )
I reported "commandant", Feb. 2017