Can/Could Will/Would Possibility/Abiltiy

I enjoy using Facebook because I could see photos of my friends there.

There is often confusion about can/could, will/would. Sometimes students learn in school that could and would are more formal or more polite than can and will.

Note: This 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

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, and so it is more appropriate 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 there is no possibility of him seeing his friend's photos on Facebook. Notice that in this example could is part of a structure called second conditional:

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.

Second conditional - an unlikely situation - is by far the most common context for could and would.

Will

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 that is highly possible:

Look at those clouds. I forgot my umbrella. If it rains I will get wet!

Predictions:

Look at those dark clouds! It will probably rain soon.

Habits (often annoying habits), and Routines:

Present habits/routines

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!
(An annoying habit..)
On a normal work day I'll arrive at work at around 8 and go straight to the canteen for a coffee.
(A daily routine. Note, however, that native speakers tend to talk about present routines using present simple tense without 'will'!)

Past habits/routines

My grandfather was extremely kind. He'd (= he would) always go out of his way to help people.
(A past habit, this time a good habit!)
When I was a student I would often go to bed late because I had to finish assignments.
(A past routine - I'm not a student any more. This is a bit like saying 'used to')

Ability

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.

Summing up

Next time you speak or write could or would stop and think. You probably should be writing can or will!