Breakdown of Hay una cosa en la mesa.
un
a, an, some
en
in / on / by (means of transport) / about (after pensar)
la mesa
the table
hay
there is / there are
Questions & Answers about Hay una cosa en la mesa.
Does hay change if there are multiple things?
Doesn't the word en usually mean "in"? Why does it mean "on" here?
Why do we use una instead of un here?
The word cosa (thing) is a feminine noun, which you can often guess because it ends in the letter a. In Spanish, words like "a" or "an" must match the gender of the noun they belong to, so we use the feminine una.