"There are many tall trees by that fence."
Translation:Annál a kerítésnél sok magas fa áll.
The main problem I see is that after an expression of quantity - in this case, sok - the noun must be singular along with its verb, if there is one; so, it would be sok magas fa van. Even if that weren't true, you're mixing a plural subject with a singular verb; it would have to be fák vannak. (But that's still not right because of the sok. It really does need to be in the singular)
áll can apparently be used to mean there are if the noun it's used with could conceivably be described as standing when stationary: trees, buildings, cars, etc. It means so many different things that I've declared it my arch-nemesis and the "get" of Hungarian. :P
I see, thank you! Unfortunately with "this" lesson I have now reached a state, where everything is just mixed up and I cannot go further with it. Too many different complicated things in one sentence. But no problems with the numbers. :-) fortunately. I Ihope, that others have less problems in conquering it.
If you can get a copy of "Hungarian in Words and Pictures", you might find the grammar explanations and examples useful. The course seems to be (generally) following the material and vocabulary in that book, at least as far as I've gone (I'm almost to the third checkpoint).