Ok, here is an example of modding English.str, fixing one of my pet peeves:
Open in notepad.
Search for 'Economic Bonus'
[GovBonuses0] Economic Bonus: 0%\nIndustrial Bonus: 0%\nResearch Bonus: 0%
[GovBonuses1] Economic Bonus: 25%\nIndustrial Bonus: 25%\nResearch Bonus: 25%
[GovBonuses2] Economic Bonus: 50%\nIndustrial Bonus: 50%\nResearch Bonus: 50%
[GovBonuses3] Economic Bonus: 75%\nIndustrial Bonus: 75%\nResearch Bonus: 75%
Remove everything after Economic Bonus: 0/25/50/75, so that it looks like:
[GovBonuses0] Economic Bonus: 0%
[GovBonuses1] Economic Bonus: 25%
[GovBonuses2] Economic Bonus: 50%
[GovBonuses3] Economic Bonus: 75%
Start a new game and take a look in the Civilization Manager to see that it works.
That example was also for any dev that comes by, since they either keep missing all the posts about the errors in the moddable files or are ignoring them completly. My example took me under two minutes to do and that includes testing so that it works. I can't help but wonder why the devs haven't fixed this in an official update ages ago.
Anyway, as my example shows, it's extremly simple to mod this file. You just need to find the text that you want to change which is easy enough, for example taking a text-string from the game and search for it in this file, or simply reading until you find it.