Plurals use the definite article die. Also, the noun changes in the plural form:
"the tool" = das Werkzeug
"the tools" = die Werkzeuge
For the singular and plural forms in all the different cases, you can check Canoo.net.
The exception to this is when nouns are used 'without plural' or 'uncountably'. Like how "equipment" in English can refer to one object or several without becoming "equipments". Werkzeug can be used this way, so you may not always see the plural form.
It's just overly-specific (and possibly redundant).
Werkzeug is not "work tool" - just "tool".
Spielzeug is not "play toy" - just "toy".
Pfanne is not "cooking pan" - just "pan".
Milch is not "cow milk" - just "milk".
Jacke is not "warm jacket" - just "jacket".
In all these examples, you need to remind yourself what is the translation, or the meaning, and what is just extra information (no matter how common it is for that extra information to be linked to it).
Strictly speaking, the plural is die Werkzeuge. Canoo.net is a good online dictionary for this sort of thing.
But Duden also mentions that it can be used "without plural", as in mehr Werkzeug habe ich leider nicht ("Unfortunately I do not have more tools"). In this way, it functions grammatically like "equipment" does in English: "This hammer and this screwdriver are my tools" (grammatically plural), vs "This hammer and this screwdriver are my equipment" (grammatically singular).
Some words are used "without plural". An example in English is "equipment". One screwdriver is "equipment", a whole box of tools is also "equipment". While there might technically be a plural term "equipments", it is not generally used.
So, das Werkzeug could mean one screwdriver, or a whole box of stuff (spanners, hammers, chisels, etc.).
If you do use the plural form Werkzeuge with a definite article (which you can do to emphasise "the [different kinds of] tools"), then yes, you would need die.
[Source: Duden German dictionary]
No. The title of the lesson is "stuff" because the word Zeug alone means "stuff". But when you combine it, it has other meanings. You need to use the correct English word when you translate, not just what it looks like when you break the word apart. If that helps you remember, though, you can use it as a mental tip. But it is not a valid answer in English.