to a high speaking score isn’t your use of ‘complex’ grammar structures, big words or idiomatic phrases. The key is to speak as naturally as possible, and real examples or stories help you to do this.
How about if I use “to have” instead of “to” in the sentence above?
That wouldn’t work. In this sense “the key to…” needs to be followed by a noun or noun phrase. You could say “The key to having a high speaking score…”, though I think I prefer the original.
Can I replace “to” by “belong to” in the sentence? Or could you suggest some alternates for this?
Do you mean “The key belong to a high speaking score…”?
That doesn’t make sense.
There is no reason to want to change the original. “the key to …” is more or less a set form of words.
This usage of “to” is new to me and I am trying to understand it. In this case, I think it is used as a preposition but can’t find it in Longman English Dictionary. Here is a list about its usage as a preposition:
to W1S1 prep
[Language: Old English;]
used to say where someone or something goes
She stood up and walked to the window.
the road to London
our weekly trip to the supermarket
sending a spaceship to Mars
These people go from house to house selling goods (=visit many different houses) .
used to say who receives something or is told or shown something
He sent presents to the children.
She whispered something to the girl beside her.
Give my best wishes to your parents when you see them.
Don’t show these letters to anyone else.
a message from the Emperor to his people
used to show in which direction something is in relation to something else
Knutsford is about 16 miles to the south of Manchester.
There was a table to the left of the doorway.
used to show the purpose, event, or activity for which you go somewhere
Sophie goes to gymnastics every Friday.
Did you get an invitation to their wedding?
Don’t forget, we’re going to a party tomorrow night.
If he needed help, Mother came rushing to the rescue.
used to say what state someone or something is in as a result of an action or change
She sang the baby to sleep.
Wait until the lights change to green.
a return to a traditional way of life
used to say that one thing is touching another
He held a knife to her throat.
They danced cheek to cheek.
used to say where something is fastened or connected
He tied the rope to a tree.
Attach a recent photograph to your application form.
Cash machines are linked up to a central computer.
facing something or in front of it
I sat with my back to the window.
We were standing face to face.
used to show a relationship with someone or something
George’s sister was married to an Italian.
He was first cousin to King Philip VI.
The robbery may be linked to other crimes of violence.
a) as far as a particular point or limit
She can already count from one to twenty.
The water came right up to our knees.
Temperatures dropped to 25 degrees below zero.
It’s ten kilometres from here to the coast.
She read the novel from beginning to end.
Does your interest in nuclear physics extend to nuclear weaponry?
b) until and including a particular time or date
They stayed from Friday night to Sunday morning.
I’ll be on duty from 8 am to 10 pm.
used to say what or who an action, attitude, situation etc affects or is related to
The factory clearly represents a danger to health.
She’s always been kind to animals.
his attitude to life
What have you done to the radio? It’s not working.
used to say who someone works for
Jane is secretary to the managing director.
used to say what something is needed for
I’m still waiting for an answer to my question.
Have you seen the key to the back door?
used when comparing two things, numbers etc
England beat Scotland by two goals to one.
Yes, she was punished, but it was nothing to what she deserved.
used to say who has a particular attitude or opinion about something
The whole thing sounds very suspicious to me.
Tickets cost £10 each and to some people that’s a lot of money.
To my mind, age does not matter; love is what matters.
used to say what someone’s reaction is when something happens
Much to everyone’s surprise she passed the exam with distinction.
I discovered to my horror that my passport was missing.
used when saying how much time there is before a particular event or time
It’s only two weeks to Christmas.
How long is it to dinner?
ten to five/twenty to one etc
(=ten minutes, twenty minutes etc before a particular hour)
a) used when talking about a rate or quantity to say how many smaller units equal a larger unit
We’re only getting 130 yen to the dollar at the moment.
There are just over four and a half litres to a gallon.
b) used to show the relationship between two different measurements or quantities
The car will do over 40 miles to the gallon.
The scale of your map is one inch to the mile.
used to say that a particular sound is heard at the same time as something happens
I woke to the sound of torrential rain.
The royal couple arrived to a fanfare of trumpets.
I like to exercise to music.
used between two numbers when you do not know exactly what the real number or amount is
There must have been eighteen to twenty thousand people at the concert.
He drowned in 10 to 12 feet of water.
(all) to yourself
if you have something or someone to yourself, you do not have to share them with other people
It was the first time I’d had a room to myself.
used to say what the chances of something happening are
I’ll bet you ten to one he’ll forget all about it.
It is used as in definition 4 in this link:
—used as a function word to indicate addition, attachment, connection, belonging, possession, accompaniment, or response
“a high speaking score” is attached or connected to “the key”