This could be achieved using gravity to allow the water to flow from the higher to the lower level.
This is possible in some languages but not in English. In English if something happens the same way, all the time, predictably, without variation, then there isn’t really any question of probability (‘could‘). For regular, predictable phenomena use good old present simple tense without modals:
This is achieved using gravity to allow the water to flow from the higher to the lower level.
Only use modals for unpredictable or uncertain situations, and then think about the degree of predictability or certainty:
This could be achieved using gravity to allow the water to flow from the higher to the lower level, but there are other, better methods. (= Gravity perhaps not the best method)
In most situations this can be achieved using gravity to allow the water to flow from the higher to the lower level. (= Gravity usually the best method)
Notice that could implies a more negative evaluation than can. Indonesians should think carefully about this distinction as they tend to over-use could, having been taught in school that could is more formal than can. Well, yes it is, but only in offers and requests:
Can you pass the salt? (informal)
Could you pass the salt, please? (formal)
Excuse me. Would you mind passing the salt? (very formal)
I enjoy using Facebook because I could see photos of my friends there.
Students are often confused about can/could, will/would. Sometimes they have learned at school that could and would are more formal, or more polite than can and will. That may be true when you are requesting something, but in IELTS speaking and writing you’re usually using can and will to communicate possibility or ability rather than to make a request.
Possibility / Likelihood
How possible or likely is it?
I enjoy using Facebook because I can see photos of my friends there.
In this case there is a strong possibility (almost 100%) that I will see my friend’s photos on Facebook. In this case I need to use can.
Let’s imagine a similar situation where there is no possibility:
If I had an Internet connection, I could see photos of my friends on Facebook.
Here the writer clearly does not have an Internet connection and so he is not likely to see his friend’s photos. Notice that in this example could is part of a structure called ‘second conditional’, which is used to describe an unlikely situation in the present:
If + subj + V2 + ‘,’ + subj + could/would + V1
In the ‘second conditional’ the situation you are describing is unlikely:
If I found a million dollars in the street I would buy a new house.
This kind of imaginary situation is by far the most common context for could and would.
Indonesian students tend to overuse ‘will‘ because they want to translate ‘akan‘. But ‘will‘ is not used in English as much as ‘akan’ is used in Indonesian. Actually there are generally only three situations where will is suitable:
‘First conditional’ – a situation in the present that is highly possible:
Look at those clouds. I forgot my umbrella. If it rains I will get wet!
Look at those dark clouds! It will probably rain soon.
Habits (usually annoying habits)
He drives me crazy. He‘ll (he will) trim his nails and then leave the cuttings all over the floor for me to clean up!
How able are you (or were you)?
It’s unusual to talk about past abilities, because once you acquire an ability, for example the ability to swim, you rarely lose that ability. It would be ridiculous, for example, to write:
When I was younger I could swim, but then I forgot how to swim so now I can’t.
We generally only lose this kind of ability when something terrible happens to us:
When I was younger I could swim, but then I lost my arms and legs in an accident and so now I can’t swim.
On the other hand ability can sometimes be a matter of degree. for example we can talk about partial ability, and about changes in our level of ability:
When I was younger I could swim 20km, but now that I’m old I can only swim 20 metres!
Most of the time when we talk about ability we use can – present tense – because, most of the time, we’re making a claim that is true now or always.
My brother is at university. He says university assignments be very stressful. He actually failed his last assignment because didn’t answer the question properly. If he misinterprets the next assignment question he fail again and this be bad for his future career.
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My brother is at university. He says university assignments can be very stressful. He actually failed his last assignment because didn’t answer the question properly. If he misinterprets the next assignment question he will fail again and this could be bad for his future career.
can - highly likely – assignments are often stressful
could - unlikely – now that he has failed once, he will be careful not to fail again
will - highly likely – the same often happens to other people