If Anna were here, she would've known what to do.
This means "if Anna were here" in a greater, pervasive sense which extends over the entire period which covers the past event during which it was important for someone to know what to do, as well as the present.
The wording suggests that the event is in the fairly recent past.
It is not interpreted as meaning "if Anna were here now, specifically at this moment, she would have known what to do in a previous moment, when she wasn't here" which doesn't make sense.
It would not be used in discussing something which happened, say, ten years ago. Then it would be
(That day ten years ago) if Anna had been here, she would have known what to do.
But imagine that the event happened only five minutes ago. Perhaps it is in a work place and Anna took the day off.
You sure made a mess of things just now! If Anna were here, she would have known what to do.
[If Anna were here (at work today) she would have known what to do (five minutes ago, when you messed up).]
Anna being here is with regard to a broader scale of time than the minute-by-minute scenario of the mishap.
In my language, you cannot possibly have some present situation that had / can have consequences in the past. How is this possible?
Translate the above to your language and you will have the answer. How do you tell someone that if Anna were here today (if she had come to work at 9 a.m. instead of taking a day off), she would have known what to do during the emergency which happened 15 minutes ago (at 1:30 p.m.)
It's possible that there is no translation to your language which specifically has that meaning, so that context is required to clarify it.
In general, if some grammatical tense distinction is missing in a language, you can make up for it with explicit indications of time or some additional context (which is the medicine that cures all sorts of semantic ailments).
Continuing with your examples:
If you wear a beard all the time, they will not recognize you without it.
If you wore a beard all the time, they wouldn't recognize you without it.
If you had worn a beard all the time, they wouldn't have recognized you without it.
If the Earth stops, everything will be changed in the world.
If the Earth {stopped | were to stop}, everything would be changed in the world.
If the Earth had stopped, everything would have been changed in the world.
Note that "be changed" is awkward, especially in this last one. For one thing, the cause of the change is clear: the change is caused by the Earth stopping. The passive voice "to be changed" is used when the agent of change is unknown, or we wish to conceal it. In this case, the natural way to express it is from the point of view that changes are taking place globally, by themselves, in response to the Earth stopping: in other words, the reflexive form of "to change".
If the Earth stops, everything will change in the world. [ Much better! Things will change, by themselves, but in response to or as a result of the stopping.]
If the Earth {were to stop | stopped} everything would change.
If the Earth had stopped, everything would {have changed | be different now}.
You ask:
but why wouldn't it be this?:
If the Earth stopped, everything would have been changed?
Because this situation doesn't support the multiple scales of time, like the situation with Anna not being here. The Earth stopping is a sharply defined event, and it is the cause of change. We regard these things as being ordered on the same time scale.
Here is how we can make it similar to the situation with Anna:
If the Earth were rotating, things would not have changed.
[If the Earth were rotating now, it would have been rotating previously at that point in time when things changed. i.e. If the Earth hadn't stopped, things would not have changed.]
This may be the key to unraveling these present tense subjunctives combined with past clauses: negate them.
For instance "If Anna were here, she would have known what to do." What is the negative way of saying "If Anna were here?" Try:
If Anna hadn't taken today off, she would have known what to do.
You ask:
If it stopped, it's not moving, then this could've caused changes in the past?
No.
The last example, in various tenses:
She will win the beauty pageant if she wears different clothes.
She would win the beauty pageant, if she {wore | were to wear} different clothes.
She would have won the beauty pageant, {had she | if she had} worn different clothes.
