I'm using Duolingo to learn Irish. One exercise I've come across that confuses me is

Translate "You want a duck"

Answer "Teastaíonn lacha uait"

This confuses me as I understand

"Teastaíonn" = Need or want
"lacha" = duck
"uait" = I need or want

So directly translated this comes out to "Want duck I want".

But I think you can also answer the question as

"Tá lacha uait"

I may be incorrect on the above for this case. But there are other cases where you can do essentially this according to Duolingo.

What's happening here?

e.g.

Translate "He does not want the hat."

Answer "Níl an hata uaidh"

Uait means neither I need nor you need but "from you" (ó + tú)

Tá X uait means "You need X" ("Is X from-you")
Teastaíonn X uait means the same. ("Is-lost X from-you")

Verbatim translations are always helpful to understand the structure of phrases.
What's from you, you can't have at the same moment, so it is absent.
And what's (unwillingly) absent, that's what you need or want.

Not the literal translations, but the effective tranlations are

Teastaíonn lacha uait. == "You need/want a duck."
Tá lacha uait. == "You want a duck."

In the case of the teastaíonn form, it can be "need" or a "want" depending on dialect or preference. (I forget which dialects lean in which direction).
In the second case, without teastaíonn in can ONLY meant "want".

A good rule of thumb for "Duolingo Irish" is that

Teastaíonn X uaim. == I need X.
Tá X uaim. == I want X.

Not literally true, but DL seems happy with that.