It’s important that you use the present subjunctive properly.
The previous sentence is an example. It’s not an obvious one, since it’s hard to use it wrong in that case. The word “use” is in the present subjunctive, but it’s the same as the present indicative in this case. let’s get a little fancier:
It’s important that a writer use the present subjunctive properly.
Here it makes a difference. The correct form of the verb is “use,” not “uses.” The sentence needs the present subjunctive because it says that proper usage is required or called for, not that the writer in fact uses it properly.
You could say, “It’s important that Tom uses the present subjunctive properly,” with the present indicative “uses.” Then you’d be saying that Tom uses it properly, and that fact is important. The two verb forms convey different meanings.
The subjunctive form sounds rather formal, so you might want to use a different construction if you’re aiming for a casual tone. “It’s important to use the present subjunctive properly” or “You should use the present subjunctive properly” are better choices, even in formal writing. They convey the same meaning with fewer words.
The important thing is not to use the indicative when you need the subjunctive. Getting it wrong can confuse the reader and look odd. If you say “I insist that you be well-dressed,” you’re telling somebody to dress well. If you say, “I insist that you are well-dressed,” you’re complimenting the person’s clothing and won’t take any contradiction.