Context and word order. Here the information that "I was here" is treated as known so in English you would stress the "in front of you"/"before you" part (i.e. pronounce it stronger than the rest of the sentence). What would you rather say this way: "But I was here in front of you!" or "But I was here before you!"?
If you wanted to say this sentence with "here" stressed, that is, assuming the whole location you were at is unknown to the person you're speaking to, you would rather say "But I was here in front of you!" than "But I was here before you!" because saying the latter this way would make an illogical connection between place and time.
A similar effect (probably - I'm not a native speaker nor a very good speaker so if somebody could confirm, it would be appreciated) can be achieved in Czech by saying "Já jsem ale byla tady před vami" or "Já jsem ale byla tu před vami" with "tady"/"tu" stressed when the whole phrase means place and and with "před" stressed if the phrase means time.
TL;DR Watch the sentence stress.
If it were "tady před vámi", then yes, that means "here in front of you.".
"Já jsem byla tady před vámi." - "I was here in front of you."
But this sentence can still be spatial. It just doesn't mean "here in front of you". It can mean "At this place, I was in front of you.". But that is quite strange and it is hard to imagine the context where it might be applicable.
To go back to the discussion about "before" and "in front of" relating to the meaning of "před", I came into this discussion because I wanted to ask A477's question. Back in the day, one finds in old English texts, "before" could mean "at" or "in" "the front of". Nowadays, "before" means "prior to" and "in front of" means "opposite" or "at the front of". Have I got this correct, that "před" could mean both but in situations like this where either could be the English translation, differentiation of which meaning it is is clear with word order? Or are there other ways of expressing these two meanings? Would it be possible to spell out how this would work in several contexts to avoid possible confusion of the student? Thanks.
I'm not sure what else can be added to what JBHayven and VladaFu have already written. It's normally clear whether "před" means "before" (temporal meaning) or "in front of" (spatial meaning). This is a rare sentence where it's potentially ambiguous. My first instinct, upon hearing/reading "Já jsem tady ale byla před vámi!" is "before you" - I was here earlier than you. One of the reasons could be that it's simply the likely meaning. And if I wanted to change the sentence to switch to the "in front of you" meaning, I would do exactly what has been suggested - place the "tady" right before "před vámi" - that would emphasize the space "here" and thus shift the meaning of před to "in front of". In the original sentence, "tady" is in the 2nd position cluster, i.e. something known or less relevant.
The whole sentence you propose still does not make sense to me. It is just incoherent. If it is correct English, then it is not the English I, as an ESL speaker, know.
Please note that the Czech sentence is just one clause , no comma is there.
Also note that "," is a comma, not a semicolon ";".
However, you can, at least colloquially, say "Já jsem byla před vámi." means "I was (here) before you.". It can also mean "I was in front of you.".