When to use "lass" and "lasst"?
I'm not sure when to use the word "lasst" and when to use "lass". For example, “Lass uns in den Zoo gehen” as opposed to "Lasst uns unsere Jacken anziehen und Ski fahren.”

When should it be "lasst" and when should it simply be "lass"? (without the letter t at the end)
What is the grammatical rule here?

Question

When to use "lass" and "lasst"?
I'm not sure when to use the word "lasst" and when to use "lass". For example, “Lass uns in den Zoo gehen” as opposed to "Lasst uns unsere Jacken anziehen und Ski fahren.”

When should it be "lasst" and when should it simply be "lass"? (without the letter t at the end)
What is the grammatical rule here?

Answer

If you address one person (singular) you use "lass" and if you address more persons (plural) you use "lasst"

Answer

In these sentences the word "lassen" has different subjects:
"lass"  (du)
"lasst"  (ihr)

You can say as well:
"Lassen wir uns ..." - "Let us..." (the same meaning, subject "wir")
"Lassen Sie uns..." - may be seen as a request, for example children ask the teacher to let them go somewhere

To understand sentences like that better, I would recommend revision of imperative and the word "lassen" (which has so many meanings in German). If you'd like some help with that, I'd be happy to assist you.  Feel free to look at my teaching profile or send me a message.