Einen is used when addressing thr tablr to somebody like wir haben einen tisch or du hast einen tisch and ein is used when just defining it like ein tisch or es ist ein tisch (only mentioning table here)
Tisch is either a desk or a table. Why is it wrong that I translate "Ein Tisch" with "A desk"?
Ein Tisch could be described as a flat pice of furniture, usually to hold things. The variations are added to. Schreibtisch is a Tisch that you write on. Like in englisch, we wouldn't translate a desk or an end table as simply a table, so we variate it. "End table" or "writing table/desk" or "coffee table."
How do we know when do we use "Ein" or "Einen" before "Tisch"? For example... you can say "du hast einen Tisch"
ein is used when the Masculine nouns are in Nominative, einen when they are in Accusative. Same logic as when der Tisch becomes den Tisch.
The first thing that comes to mind when reading "table" out of context is "furniture-table", rather than "chart-table", so they probably just didn't think of "eine Tabelle". Still, it is a valid translation, so report it next time.
"Ein Tisch" means "A table" right? It corrected me and said it was "one table"
It means both, and both are accepted. Are you sure there weren't any other errors/typos in your answer? ‘A table’ is the suggested translation, so, barring a bug, it's weird that Duo would correct you on that.