"Good day, I am a beginner."
Translation:Bonan tagon, mi estas komencanto.
I'm merely a beginner, but I can try to give some insight to a fellow learner. I shall point out that Tagon is the accusative form of tago (use tagon if it is the object of something). Tago by itself is nominative and would be used as a subject of a sentence and not an object. One does not use the -n suffix if the noun is a subject, but one would use it if said noun is a direct object.
I can give two examples: 'La tago estas longa', which means 'the day is long'. As you can see here, tago is the subject of the sentence and does not have the -n suffix attached to it; therefore, tagon (the accusative form) can not be used if the noun is a subject.
As opposed to: 'Mi donacis faston al la tagon', which translates to 'I gave (or donated) a fast to the day'. As you can see here, tagon and faston are the objects of the sentence (mi is the subject); therefore, tago and fasto aren't valid choices in this instance, for it would be impossible to see the difference atween the subject and the object of the sentence without understanding the grammatical functions of words if a suffix wasn't attached to it (it actually makes it easier to understand the grammatical function of a word, thus preventing the difficulty of telling how a word functions).
Anyway, I am glad to be of help. You have chosen to learn a wonderful tongue with a beautiful and sheen sound; for that, you have my respect.