Misintpretation
Ok, so this might sound trivial but here goes.
Lets take the phrase:
come un pescado. (he eats a fish). How do we know WHO is eating this **** fish??
It could quite easily mean, she eats a fish, or even you (formal) eats a fish. Surely in this sentence you would need the use of a pronoun to identify who's doing what!?
Likewise, If was to stop someone walking down the street in mexico and said
"Habla Ingles?" - do you speak english? (you being formal as I dont know them).
This could also mean he/she speaks english which is a STATEMENT and not a QUESTION. In other words, if I was with a friend who doesent speak spanish and the mexican spoke to my friend, I could use the same part of the sentence - No habla espanol. (Meaning he (my friend) doesen't speak spanish. But could also double up as you formal dont speak spanish?
CONTRIDICTING?!?!
7 Answers
The thing is that sentences are usually written in context or with pictures if there's little or no context. Where would you find the sentence. 'He eats fish.'? Usually straight after some other comment about Johnny, Juan or some other guy so you'd know it's a 'he'.
The same with speech, it is usually accompanied by some kind of gesturing as well as voice inflection. The person would know if you were asking a question if your voice went up at the end of the sentence and your eyes (at the very least) would indicate that your question refers to whoever you're looking at. Also it'd be pretty strange if you walked down a street, stopped a stranger and went on to inform him that your friend doesn't speak Spanish! 
The mere inclusion of a pronoun is hardly sufficient to make the meaning of a sentence clear in all cases. If you were to walk up to someone on the street and announce (out of the blue) "She speaks Spanish." The hearer might very well wonder "She? Which "she is that?" (given that "she" could apply to any one of roughly half the people in the world).
Probably most of the sentences that you say, if taken out of context, would be unclear.
You usually understand the meaning by the context...
Yes... it causes mistakes... as in..
"su pluma"... it could mean "his pen", "her pen", "your pen".
So you'll have to do as we do... ask....
¿de quién?... de ella
Misintpretation
It is closely related to how everyone understood what this thread title meant.
How often do you hear abbreviated sentences in English, and have no trouble understanding?
Wanna eat?
Yeah, going soon.
Didn't know that!
Okay, got me there.
You innately understood the dialog above, but analyze it the way a student of English would, and you see it is the same situation you complain about. Just get used to it.
Hi, welcome to the forum. You may want to read forum rules before you start.. Just in case..And don't worry. Everybody is going to understand what you are trying to say if you do it connected to the context. Even the way you say it will make people understand it, for example your voice. And when you think that the sentence you are going to say will confuse people , then use a subject, it's not forbidden.. But generally it's not used exept of some cases. If Spanish still exists, it means there is no wrong part, does it not?
There's always one smarta**e isn't there. Traffic warden lol.
