Help with English

Achilles was an Ancient Greek hero, who fought and died during the Trojan war.
according to some reports he was shot by a poisoned arrow in the heel, leading to his death.

The story goes that when he was born it was foretold that he would die young, so his mother, holding him by his ankle, dipped him into the river Styx, which was supposed to grant invulnerability.
However, as she was holding him by the heel, his heel was not covered in the water so became his weak spot.

The term "Achilles heel" now refers to a weak spot in an otherwise strong position.
I think it was Coleridge who once referred to britain as Achilles and to Ireland as its vulnerable heel, so bringing the metaphor into common use in the early 1800s.
is gang bang vulgar?
Given what it refers to, even if used as a metaphor it is not generally a word I would use in polite company.
brother, father, mother,
but sister, why not sisther?
Father comes from the old Germanic word "fader", mother from "moder", and brother from "broper" (although the "p" is a different pronunciation to the modern "p").
These developed, in English, to the modern spelling and pronunciation during 1500s or so when the "d" or "p" sound was starting to be replaced by "th".

Sister, on the other hand, derives from the old Germanic word "swester", which gradually changed to "sister.

So while your question is valid in that you're looking at the words as being linked through meaning (of family members) and hence might expect a common approach to spelling, their etymology is the key to understanding such things.

I want him report/reports to me as soon as possible.

which one?
You want him to report to you as soon as possible.

With the verb "want", one either wants an object or wants something to happen.
So while you may want a banana, want an icecream, want top marks on your exam, when it comes to wanting actions to happen you might want to study, want someone to report to you, want her to go out with you, want the dog to not foul the pavement etc.
It doesn't mean the same thing. Runner up is only after third place, it's like not winning but almost.
It doesn't mean the same thing. Runner up is only after third place, it's like not winning but almost.
As I understand it it does mean the same thing...
First runner up is the same as second place.
Second runner up is third place.
why say first runner-up, second runner-up?
why not just say second place, third place ?
"Runner-up" is usually given to the person who loses the final in a knock-out tournament, such as tennis.
But it is now also generally used to describe the second place of any competition.

No idea what the origin of the idiom is.
You could also call the second-place person the "first loser" etc.
It's probably because everyone wants to feel like they've done better than they have... so first runner up might sound better than second.
I have many neighbors.

neighbor is countable?
"Many" is uncounted, not necessarily uncountable. I can say I have "many" cousins but I can't tell you how many without listing all of my parents' siblings and listing all of their offspring. Saying "many" can mean that I haven't counted them yet or it can just mean that the exact number isn't important. Or it could mean that I just don't know.
I think he was asking if neighbour is a count noun - i.e. able to be counted.
To which the simple answer is yes.

The general rule is that for count nouns you would use "many" and for non-countable nouns you use "much" etc.
Yes, there are exceptions, just as there are for the similar difference between "few" and "less".
1. We live to eat.
2. We eat to live.

What's the difference?
1. expresses that the (primary) purpose of living is to be able to eat.
2. expresses that the purpose of eating is to be able to live.

It's the difference between whether you make eating a focal point of life, enjoying it etc, or whether it is more a functional requirement of your day, merely something you do so that you stay alive (to do other things).
Thirteen people died and dozens were injured when a van ploughed into crowds in Barcelona's Las Ramblas area on Thursday afternoon. The driver of the van fled and is still at large.

still at large
means what?
Got "still at small" ?