"I am outside."
Translation:Tôi đang ở ngoài.
I'm also learning, so not sure, but here's a rule I think works so far for how to translate "to be":
Use làfor "Noun is Noun": I am a student She is my friend Your name is Grokford
Use đang for "Noun is Verb-ing" (i.e. to make progressive tense): I am singing
Use ở or đang ở for "Noun is at Location" (not sure of the difference btw ở and đang ở): I am outside
Use (nothing) or rất for "Noun is Adjective": The cat is big.
Treat phrases in ... I don't know how to say this ... the correct way. For example: I am a green cat should use "là" not (nothing) because "green cat" is a noun phrase -- it should be treated as a noun, not an adjective.
A special case: in Vietnamese colors are nouns, not adjectives. So you have to say: The cat has brown color. and not The cat is brown.