Hi Konrad! I have highlighted below some mistakes and added in bold missing words. You should also work on your punctuation (which I did not correct here). If you need any further help please do not hesitate to message me! :)

My article about the Roman Empire - part 1 (preface)

My article about Roman Empire - part 1 (preface)

Read the following link on when you should use "the"

https://www.engvid.com/english-resource/the-with-country-names-lakes-rivers/

We should learn from historical events and extrapolate from them the effects of actions we undertake.
The greatest state in the ancient era had lasted for almost ten centuries. To imagine how long that period of time is we can think about Poland one thousand years ago. Bolesław Chrobry was the king Poland and the Polish state just starting to be formed. For us such time is almost unimaginable. Yet, Rome collapsed, after decades of internal struggles and uncounted smaller and bigger wars with barbarian invaders.
One of the many root causes standing behind the fall of the Roman Empire was extremely important, because of its vast impact on the other factors. That cause was strictly economic one and it consisted of various elements such as currency depreciation, hyperinflation, soaring taxes, growth of the public bureaucracy prompting the need for even higher taxes, depravity of the emperors who introduced more and more calamitous regulations, crumbling trade as the effect of the coinage debasement and at the end, the whole disintegration of the contemporary financial system. Along with the deteriorating financial condition of the Roman State, military necessities had not been lower. The growing threat of the Persian Empire on the east, barbarian tribes on the Rhine and the Hunnic wave storming through the European continent required a  large and strong army with high morale. The army in the 4th and 5th centuries had known its importance for the stability of the whole state, thus soldiers’ financial demands were also expanding. But the authorities were not able to meet their demands and the effect of this was disastrous for the integrity of the Empire, because of the never-ending need for new recruits, multiple civil wars and riots with self-proclaiming emperors-generals. Hence, from the one side there was a worsening financial condition exacerbated by central economic planning and from the other – rising and never acquired military needs.
The Roman Empire fell because of its military weakness, which is true, but the reason for this lain in the financial system and money. At the beginning of the 3rd century the Roman state entered into fatal an economic spiral which had lasted till the end of its western part in 476 A.D. But deterioration of the empire’s economy had begun already in  the 1st century during the reign of Nero. To understand why we need to start with the Roman currency system.

It's an interesting period of history but few know much about it. You wrote a good text, but be careful to include the definite article "the" when required. See the following link

https://www.englishclub.com/grammar/determiners-articles.htm