Helen will feel better if she swallows this pill.

CORRECT. Swallows is in the present tense of the indicative mood. The presence of will in the clause She will feel better requires that the if-clause be in the indicative mood. In other words, the verb in the if-clause takes the indicative mood because, as the use of will demonstrates, the author is at least neutral in his or her beliefs about Helen's chances of swallowing the pill.

Can somebody explain the jargon's used above in a simple user friendly language ?