Breakdown of Czy tu jest drzewo? Nie, tu nie ma drzewa.
być
to be
tu
here
czy
[question particle]
mieć
to have
Questions & Answers about Czy tu jest drzewo? Nie, tu nie ma drzewa.
Why does drzewo change to drzewa in the second sentence?
The first sentence uses jest (is), so drzewo stays in its basic dictionary form. The second sentence uses nie ma (there is no), which always forces the following noun into the Genitive case. For neuter nouns ending in -o like drzewo, you change the -o to an -a to make it Genitive.
Why do we use nie ma instead of nie jest to say "there is no"?
In Polish, you generally cannot use nie jest to say that something isn't there. You have to use nie ma, which literally translates to "does not have." Whenever you want to express the lack or absence of something, nie ma is the required phrase, and it always takes the Genitive case.
If nie ma means "does not have", who is doing the "having" here?
Nobody! It is an impersonal phrase. When used this way, nie ma functions exactly like "there is not" or "there are not" in English. It simply points out that the tree is absent from this location.
What does Czy do at the start of the first sentence?
Czy is a question word used for yes/no questions. English asks a question by flipping the word order to "Is there...", but Polish keeps the normal sentence order (tu jest drzewo - there is a tree here) and simply puts Czy at the very beginning to signal that you are asking a question.