"This beer is strong."
Translation:Dieses Bier ist stark.
You know how the word for "the" has to match the gender of the noun?
der Mann = "the man"
die Frau = "the woman"
das Auto = "the car"
Well, the word for "this [something]" has to match the gender too. You will notice that the endings are the same as for the forms of "the".
dieser Mann = "this man"
diese Frau = "this woman"
dieses Auto = "this car"
One thing that is confusing at first is that there's a difference between "this [something]" and just "this" without a noun after it. If you don't have a noun after it you can use dies, regardless of the gender of what you're talking about. If you said the sentence Dies ist stark! you could be talking about anything.
If you use the form of "this" with endings without a noun, e.g. Dieses ist stark! then it can only refer to something with the matching gender (here neuter). Translated, it means something like "this one". It has to be clear from context what you're talking about, otherwise (as in English) someone is likely to ask "this what?".
This is a different usage than the last lesson. "Dieses Bier" needs to be declined because you're using "this" as a pronoun in place of "the" so you need to end it in "-es" to show that the noun is neuter.
In previous lessons we were using usage more like "this is beer" so you could say "Dies ist Bier" because the pronoun is generic and not belonging to the noun itself.
You could say "This is beer, which is strong" as "Dies ist Bier, das stark ist" as an example of how you could convey similar meaning with "dies", but as you can see that introduces its own set of complications.