"The cat climbed the tree."
Translation:Katten klättrade upp i trädet.
Good question! The answer is yes. When climbing the tree, you're considered to be i trädet since you are inside the canopy, climbing the branches. But regarding the mountain you're considered to be på berget since you're climbing on the outside of the object.
How and I EVER going to remember all the little prepositions that need to go in these sentences. Isn`t "upp" enough?
Just want to point out that up is not a preposition in English either, it’s rather the verb to climb that is constructed without a preposition. You can say He climbed the tree or He climbed up the tree, but in Swedish han klättrade upp för/i berget/trädet.
Of course "up" can be a preposition in English (as well as a whole bunch of other things). If it's distinguishing between climbing "up the tree" or "down the tree" those are both prepositional phrases. Both can be moved around as a unit. Though you probably wouldn't, you could say "Up the tree he climbed!" "Down the tree he climbed!" and it would not be incorrect or unintelligible. But it's rather splitting hairs (or irrelevant really) to say whether a particular word is a proposition or a particle in any given case. I was just lamenting that it's really really hard to remember what all I need to put in in Swedish.
Just to be clear, "up" (in English) has a variety of functions, including use as a preposition:
1 From a lower to a higher point of (something)
'she climbed up a flight of steps'
'She pushed her glasses further up the bridge of her nose.'
'We picked our way up one side of the ridge, and I found a spot where we could spend the long night ahead.'
'I don't know where we were, but I wanted to climb up a really steep hill - which seemed to take ages.'
'She shrieked with laughter as they raced up the stairs.'
1.1 To a higher part of (a river or stream), away from the sea.
'a cruise up the Rhine'
2 Along or further along (a street or road)
'he lived up the road'
'walking up the street' https://en.oxforddictionaries.com/definition/up
Turn the situation around. Consider yourself a Swede trying to learn English prepositions. We are just as confusing. That helped me.
uppåt is typically used as a general direction, not with an object. There are idiomatic exceptions (e.g. helt uppåt väggarna = ludicrous), but usually it doesn't work like that.
Why is the English phrasing not "the cat climbed up the tree" to match the Swedish and hint people about the need for "uppför"?
The default is upp i, it's just that uppför is also acceptable. Unfortunately, Duolingo's system tends to show correct solutions which should be accepted but are not actively taught in the course.