It's a (demonstrative) pronoun when there's no noun after it: Das ist groß!

It's a (definite) article when there's a (neuter) noun after it: Das Wasser

The word dass is a (subordinating) conjunction, and is at the beginning of a subordinate clause.

Very briefly put: das = "the" or "that/it", dass = "that".

Due to German punctuation rules, you will always find a comma before dass, as it indicates the start of a subordinate clause.

If someone uses "Das", is there a way to know wether they mean 'that' (as in "Das ist groß ") or if they're talking about the article (as in "das Wasser")?

Das before a verb means "that/it". Das before a noun (which is conveniently always spelled with a capitalised first letter) refers to the article.

If you can exchange "that" by "which" or "who", by "it" or by "this" it is "das", if not it means "dass". In German we have the rule "Das s im "das", es bleibt allein, passt "dieses", "jenes", "welches" rein." (If you can exchange "das" by "dieses", "jenes" or "welches", it has just one s.)

I think after reading (and understanding) all these comments you will make less dass/das-mistakes than an average German.

By the way: When you read German texts you might sometimes find the word "daß". This is the old version of "dass". There was a orthography reformation in 2006.

Just to give an example of "that" being used as a conjunction in English:

"It was so hot that I took my shirt off."'

The "that" in that sentence would be translated as "dass."

(I needed an example to first understand the difference. Hopefully that helps!).