People from the past arriving in the present, even if they come from a time before English exists at all, they will be able to communicate perfectly with 20th century people. Of course, there will probably be a few “humorous” incedents where they encounter modern technology.
Conversly, people from the present who travel to the past don’t have the problems you’d expect they might. They can speak the local languange* and survive just fine despite conflicting styles of dress, ignorance of local laws and customs, having no acceptable currency, no form of travel, no place to stay, a completely different set of diseases from the present which they are not immune to, and host of other problems that should arise. They will also be able to use 20th century knowledge to recreate items using supplies in the past, no matter how dumb they have previously shown themselves to be.
*Which is 20th century English, of course, unless it’s Medieval Europe, in which case it’s 20th century English trying to sound Shakespearian with lots of thous and wherefores and all tossed in.
If you change the past and improve life for one of your ancestors, all subsquent generations will be exactly the same, just more successful. Despite having radically changed their future, they’ve nevertheless managed to procreate at the exact same times they did before, right down to the same sperm! They’ve also chosen the same names for their children as they did before.
When dealing with alternate universes/timelines, the “you” in the other universe will look exactly like you and have the same name you do, unless they’re of the opposite gender.
It’s really easy to suddenly attend a high school under false pretenses. You don’t need any transfer forms or identification for your new identity. Just show up and they’ll let you in, no questions asked. If you’re much too old to be a high school student, like 25 or so, don’t worry, no one will catch on.
You also needn’t worry if you’re trying to pass yourself off as a member of the other gender or a different ethnicity. No one will notice that, either, until you tell them. Be warned that there will be a situation where you must alternately switch between your real self and adopted persona in rapid succession trying to deal with two separate people wanting to see both of them simultaneously. There will probably be a point where you have forgotten to remove an item of your disguise. Be careful the person expecting you does not see it.
But you still won’t be able to do this until there is only one second left. Exception: James Bond can defuse a bomb six seconds faster than anyone else.
In the unlikely event they do, it’ll probably be someone they feel responsible for taking care of, like a younger brother. This person will die, either accidentally because of something a good guy does (if they’re an innocent), or because they tried to kill the hero or hero’s friend themself against the villian’s orders (if they’re a hothead). Either way, the villian will become extremely enraged and irrational for the remainder of the film, no matter what their previous demeanor normally was.