The word 'GRAMMAR' has a great capacity to frighten people. Many otherwise sensible people worry far too much about their grammar.
As a rule, you're likely to make grammar mistakes for just three reasons:
Most grammar errors in business and academic writing occur in long sentences. So here's the best tip of all: keep all your sentences under 21 words. In a short sentence it's comparatively easy to control the grammar.
Who cares if you can't tell a gerundive from a geranium? That esoteric knowledge doesn't improve your writing one jot. Think more about writing clearly, and less about grammar. You can usually dodge the danger zones by rephrasing.
You need to know just enough grammar to:
The following errors are very common, and they get noticed. Do you habitually make one or two of these errors? Then you just need to change one or two habits. Is that so hard?
Above all, be aware that grammar is not a religion or a fixed set of rules. It's merely a guide to current conventions. Many 'rules' vary according to country, industry, time, and style guide—so follow your organisation's style guide.
(Crash Course in Corporate Communications by Rachel McAlpine has more detail about these common errors. The book is now being radically revised to reflect business writing in the digital workplace.)