Do you put an an after the word euglena

I'm confused. It makes more sense if you have an a instead but the word Euglena starts with an e.

Because euglena isn't a verb, you generally don't put either "a" or "an" after it.

If you mean *before* it, on the other hand, you should use "a euglena" rather than "an euglena". As for exactly why, I'm not sure I can explain that. Other words that start with the "you" sounds, like YouTube, also use "a" as their article, even though the first sound of the word is essentially the same as a long "e" sound, which is generally preceeded by "an" (e.g., "eerie", "eastern"). The only certain things in English are that it's confusing and that there are exceptions to almost any rule.