Damir070889, Thank you for posting in the Intel® Communities Support.
Just to let you know, we only provide support in English language and currently, we are using a translation tool.
In reference to your questions, "Does this work? won't anything happen? won't something burn?":
The Memory RAM supported by the Intel® Core™ i5-10400F processor is, as you mentioned, DDR4-2666, which should be the proper memory RAM for your system:
https://ark.intel.com/content/www/us/en/ark/products/199278/intel-core-i510400f-processor-12m-cache-up-to-4-30-ghz.html
It is not recommended to use a memory RAM that is not supported by the processor. Still, there are some users that decide to use it outside specifications, and it will work, but sooner or later, you might start noticing the system's instability since the processor is running at unsupported speeds that might hurt the longevity of the unit and the rest of the components in your PC.
Any questions, please let me know.
Regards,
Albert R.
Intel Customer Support Technician
