"If you speak English, I cannot understand that."
Translation:Se vi parolas angle, mi ne povas kompreni tion.
The original sentence is "I CANNOT understand that", so you need to add the verb "povas", which indicates the ability to do something, before the verb "konprenas". Also, you need to change "konprenas" to "konpreni" (the infinitive form of "konprenas") since it immediately follows another verb.
I hope that helps ;)
The sentence has two errors: You do not just say "anglan." You must either use the adverb "angle" or the object-noun "la anglan". You must include the article "la." Secondly, as lingo928 pointed out, the phrase includes "cannot understand", which is "ne povas kompreni." However, I think it is perfectly fine Esperanto to just say: "mi ne komprenos tion" (future tense: I will not understand that).