Translation:If I had a username, I could log in.
People use the a/an rule based on the sound the first syllable of a word begins with, not the actual letter it begins with (as you might have learned in ESL classes). Username starts with a 'y' sound (the same as in yellow or yak), and the 'y' sound is considered a consonant in English, and so we use 'a' rather than 'an'. An username would sound unnatural, because it's like you're saying, "An yusername'.
You'll see the same thing happen in American English with the word 'herb'. Americans drop the h and pronounce it 'erb', and therefore they say 'an herb' (an erb) rather than 'a herb', even though the second, according to a strict interpretation of the a/an rule, should be correct.
Oh! I think I got it... So, when the letter works as a semivowel, we don't use "an"... Dankon!
That's interesting, very glad to learn it, but I think the awesome people who make that course for free are being a bit too much annoying with english grammar and spelling mistakes.
If they didnt, then this question never would have been asked and this learning oppurtunity wouldn't have happened.
To be fair, the primary purpose for this course should be learning Esperanto, not learning English.
I keep writing "If I had a username, then I could log in.", I keep getting marked wrong, and I keep reporting it.
It's a valid translation.
You can either “log in” or “log on” when you begin a computer session. The American Heritage Dictionary of the English Language (4th ed.) defines the two verbal phrases this way: “To enter into a computer the information required to begin a session.”