Helen would feel better if she swallowed this pill

Explanation. Swallowed is in the present tense of the hypothetical subjunctive mood. The presence of would in the clause Helen would feel better requires that the if-clause be in the hypothetical subjunctive mood. In other words, the verb in the if-clause takes the hypothetical subjunctive mood because would indicates that Helen is unlikely to take the pill.

Doubt: I don't get the Jargon's used above. Can somebody explain the same in more user friendly manner?

Also what would be wrong if i were to rephrase the above sentence as -

Helen would feel better if she swallow this pill