Accusative object & singular - looks like plural, but isn't one
Some words look like plural, but they are not. For instance: co-worker = der Kollege (sing, & nom. object) - den Kollegen (sing, & acc. object); astronaut = der Astronaut (sing, & nom. object) - den Astronauten (sing. & acc. object); boy = der Junge (sing, & nom. object) - den Jungen (sing. & acc. object); gentleman = der Herr (sing, & nom. object) - den Herren (sing & acc. object).
Note that most online examples won't highlight this confusing fact, because some nouns may not look like the plural form as acc. objects, e.g., der Mann = den Mann. This online examples has an acc. noun in the sing. form that looks like a plural. http://online-lernen.levrai.de/deutsch-uebungen/grammatik_5_7/01_nomen_grammatik/22_uebung_vier_faelle.htm
Some masculine nouns, called weak nouns, have a special type of declension that adds an -n or -en to the word outside of the nominative case. This applies to nouns with particular endings and also a few random words that you unfortunately just have to memorize. Here is a great summary that should help you out: http://www.vistawide.com/german/grammar/german_nouns03.htm
Only one problem now; How do you know which nouns will add an 'n' or an 'en' in the accusative case?