Learn Spanish (Spain): Escribo con un bolígrafo negro.

In Spanish, the ending of the verb tells you who is doing the action. The -o at the end of escribo means "I write". You can say yo escribo for emphasis, but it is usually left out because it is already understood!

Why is it bolígrafo negro instead of negro bolígrafo?

In Spanish, descriptive words (adjectives) like negro (black) almost always come after the thing they are describing. So instead of "black pen", you say "pen black" (bolígrafo negro).

How do I know to use un instead of una here?

Bolígrafo is a masculine noun, which you can usually guess because it ends in -o. Because it is masculine, you must use the masculine word for "a" or "an", which is un. If the noun were feminine, you would use una.

Could escribo also mean "I am writing" right now?

Yes! In Spanish, the simple present tense (escribo) is often used to talk about actions happening right now, as well as general habits. So this sentence can translate to "I write with a black pen" or "I am writing with a black pen" depending on the context.