"She saw a tall man yesterday."
Translation:Ayer ella vio a un hombre alto.
Just to explain it a little more, the personal "a" is used whenever a person or a group of people receives the action of a verb. The "a" goes before that noun (or if the noun is preceded by la, las, el, los, un, una, su, sus, tu, tus etc. it goes before that). This can also include pets since we almost think of them as people.
I'm not native, but since I speak portuguese, which is close enough, I might anwser: "alto hombre" is tecnically correct, but extremely unsual. If you ever use it, they'll understand what you're saying, but it sounds scholar, or literary, like if you were speaking the spanish equivalent of Shakespeare. So, a native would probably consider it funny, but would understand you. In romance languages (like spanish, my own, or french), putting the adjective before the noum is reserved for restricted cases and normally slighty changes the meaning or, in this case, the mood.