Translation:The birds eat all the strawberries.
This is unfortunately not quite true. You can use either the definite or the indefinite after alla, depending on the situation. If you'd use the definite in English, I would prefer the definite in Swedish - and if you'd use the indefinite in English, I would prefer the indefinite in Swedish. There are definitely situations where one is much better than the other, or the only correct one.
Is there that big of a difference between äta and äta upp? Like in English we could say "He eats all of his food" or "He eats up/finishes all of his food" and neither would be particularly different.
Of course, there would be a difference between "He eats his food" and "He eats up/finishes his food" (albeit not a large difference), but in english I feel like "eats all" means almost the same thing as "eats up all/finishes all". Is it this same with "äta upp alla" and "äta alla"?
Good point, although I think a closer analogue in English is the phrase "gobbles up". We would be far more likely to say "Mary gobbles up all the ice cream" than "Mary gobbles all the ice cream."
With eats, although you can add up, it doesn't feel like it's required. ("Mary eats all the ice cream, so I never get any" would work equally well, at least to my ear.) I suspect that is partly because if we just say "eats" in English, it would not be equally likely to be understood as meaning "is eating".
Here "alla" is followed by a noun in the indefinite plural, and apparently translates to "all the". But I could've sworn I've come across this particular sentence on Duo: "Hon har alla väskorna" (She has all the bags), where the noun is in the definite form. So why the difference, could anyone explain?
I am confused about when to use indefinite plural and definite plural after "alla". In this example, we use jordgubbar, i.e. the indefinite plural.
But in another example ( Älgarna äter upp alla äpplena i trädgården) we use the definite plural for "alla äpplena". Is there a rule or logic to these differences?
This is another phrasing that's similar in English Yorkshire dialect, we use "eat up" (even in South Yorkshire sounding like 'ate upp') and sup up to mean eat and drink until finished, respectively.
It seems the emphasis here, as in Yorkshire, would be that you're stressing the absolute totality of the situation.
In fact the phrase makes it into the Yorkshire anthem, "On Ilkley Moor baht 'at" but here it's the ducks that ate up the worms... http://lyricsplayground.com/alpha/songs/o/onilkleymoorbahtat.shtml
It confuses me a little this phrase. I find that "äter upp" and "alla" are redundant, they both express that all of the fruit is being eaten. Am i wrong? Why is it that way?
Also i find the "äter upp" phonetically similar to "eats up", does "x upp" is a similar construction than in english? like "finish up" or "clean up".
I don't see why "the birds eat up all the strawberries" or "the birds are eating up all the strawberries" wouldn't be accepted here - especially given in many of the comments below we are using 'eat up' and 'eaten up' in English to explain this phrase. It's acceptable, even common in English to say.