with sky portal trick

ok, but not natural light source, it doesn't look like sunlight.

with sun (+exposure)

so beautiful light, but can't remove the FIREFLIES. SO MUCH FIREFLIES.

tricks do not work, it happens only with sunlight.

with spotlight

looks good, even not so beautiful like sun. fireflies can be reduced with more samples.

Check front(numpad 1), right(numpad 3) and top(numpad 7) and check if it is on a good place.

Pink range is bad place to place light, but sometimes seems dark even if it is upside.
But it's ok, when you intended to let frontside dark(This tip is not for them!)
If you want to let light backside but want to see also bright frontside, you can use two or more lights.(see 3)
IT DOES NOT APPLY TO TRANSPARENT OBJECTS(also glass bsdf)

It looks bright when it is between camera and object, but object can reflect the light.

Bonus Hint. Translucent BSDF is opposite!

2. Change Light Strength ***

Select your light, and set light strength. You can change it to 1000, 2000, or 10000, just try until you are satisfied.

Tip. when light size is big, it reduces white points(we call it 'firefly'), but also brightness reduces.

3. You removed light!

Be careful. When you select object(like b key), it is usual to lights also are selected (and when select all, also camera). Don't worry, go to Add > Lamp and select one. (do not need to hesitate, anyway you can change light type at any time!)

4. Because background color is grey

It reflects(glossy, but also diffuse) or refracts(glass). Try to change background and see how it works. It will be many ways to solve the problem, like changing materials(roughness, using nodes and etc) or change light paths settings on the rendering option.

If object reflects too much background, use world's node, set strength to low value, and try 2(change light strength).

I believe that it will be useful for newbies. Don't forget it.

2. Check if it has extra surface

If it has extra surface, you need to remove it.

You can check by using ‘h’(hide) and ‘alt + h’ (stop hiding) in edit mode.

(faces + h key)

(select and remove face)

(ctrl + h to close, it, I forget to take picture after remove)

3. Use ‘edge split’ modifier

It will be work when it has simple surfaces. If it is complicated mesh, you need to try another one.

4. If you have problem with hole subdivision

This will be work when you cut surface using 'project knife'.

The best resolve is removing faces and rejoin them. Because of this it is recommended to use simple mesh and use subsurface. It will be useful to correct when you meet with other problems.

Even if it is not perfect, still looks quite better.

And you’ll get better result with bevels to circles (1).

1. When 1(bevel) not work

You need to bevel with edges on the faces, which is connected to the hole.

5. Just cut your cake

Yes, knife tool(k) sometimes makes magic.

6. Has it unmerged point?

If it looks like this, it would be.

Just try moving corresponding(or its neighbor point) point in edit mode and check it!

If it moves like this or you found that it hasn't been merged, select other unmoved point which was in same place...

and ALT+M and select AT LAST POINT

7. extrude and close

this works good with cylinder objects.

alt+s select > press e (extrude) > press s (change size) > fill (f) > lift up a little bit (g key and z key)

Works same like 1.

8. Surface is reversed

If it looks like this, in edit mode, select surface > Ctrl+N.

Remember that surfaces must be organized!