Long time ago I shoot still life with two halogen lamps with self-made paper diffusors, and results was very good for me.

Now I have one flash - Profoto D1 500 Air with 40x60cm softbox.
I try to shoot with it for about 6 month, but I don�t like my results.

Can you please recommend, where can I reed about shooting still life with one softbox?
Or can you recommend some lightning setups with one softbox only?
Can I shoot isolated objects with it? Isolated on white or isolated on black?

Or it is worthless and it is better to sell it and get two cheaper flashes?

I have this "less is more" mentality in the studio and have been having fun with playing with one softbox.  It really depends on what you are shooting and the look you want. As you probably know not all concepts can be pulled off with one strobe, but you can get some nice stuff with a single strobe. I'd just Google strobe use as there is plenty out there on lighting.  I will tell you that one thing I did years ago was to allocate some time to "playing" in the studio, trying new things. Here in Huntington Beach we often have overcast days. That's a perfect natural softball where you could use your light as a highlighter to a subject on, say, a table. It's a little more work to move stuff outdoors, but it helped me for quite some time.

With one lamp you can do a lot of things if in addition you use reflectors like white paper or mirrors (I use a lot the cardboard that you can find in some pack of salmon, with one side gold and one side silver - you are in Kiev, I think that you understand what I speak about ).
In this (attached) photo the phones are illuminated by one lamp, direct from back-left (the one making the blink on the top left of the phones). All the frontal light on the phones comes from a white paper reflecting the light arriving from this lamp. Nothing more.
The light on the background, placed 10 meters behind the phones, has no influence on the phones, so speaking about the phones we can say that they are illuminates with only one lamp.

To arrive to good results you have to try, and try again, move your lamp, go around the object and look the effect of the light on it.
After a while you will be able to place your lamp in the right place to obtain the result you want to obtain. For this you need time, I could say years, before to be able to master your instruments perfectly.

Look what do other photographers, try to understand how they do it and try to make a photo giving similar results. It is very better than to read books or internet sites giving you ready solutions without give you a full understanding and feeling of the light�

Just open your eyes and look around you� (the sun is a unique light making the most beautiful existing light�)

You can bounce light in all sorts of ways, it's trial and error, the best lighting will vary from subject to subject. If it pleases you, it is right (or, at least, as right as you can manage with yout current vision).

Oh ... just read a previous comment,  the sun is a very harsh light, a point source. Diffuse light is usually more endearing. Bouncing diffuses.

You can bounce light in all sorts of ways, it's trial and error, the best lighting will vary from subject to subject. If it pleases you, it is right (or, at least, as right as you can manage with yout current vision).

Oh ... just read a previous comment,  the sun is a very harsh light, a point source. Diffuse light is usually more endearing. Bouncing diffuses.

+1
I completely agree with you when you say "If it pleases you, it is right".
I have always thought that the good image is the one I am satisfied with, at first.

When I told about sun I did not meant direct sunlight, I meant any kind of light that can be produced by the sun in different circumstances, from direct sun with clear sky to sun covered by clouds, and using reflectors or diffusor panels - many photographers have worked in this for years btw