Konditional I.

This mood expresses a supposition or condition. It can be translated as "would" + verb.

Formation:

würde, würdest, würde, würden, würdet, würden + Infinitiv.

The conditional is used as an equivalent of the English form with would, i.e. to express a condition, a wish and a supposition. To express a condition and a supposition, Konditional I or Konjunktiv II are used when the condition is present or future (as "would" is used in English).
Konjunktiv II is usually used for sein, werden, haben and modal verbs. For other verbs, Konditional II is usually used (corresponding Konjunktiv II forms are used in the sublime and archaic language, additionally - they are often identical to the past tense forms).

So we'd rather say:
Wenn ich gesund wäre, würde ich nach Berlin fahren.
If I were healthy, I would go to Berlin.
The first verb is sein, so we'd rather use Konjunktiv II (wäre). The Konjunktiv II form of the second verb would be führe, and we would probably use it in solemn language, but in colloquial speech we will use the Konditional I.

Similarly :
Wenn er schwimmen konte, würde er ihn retten.
Können is a modal verb, so rather a Konjunktiv II. The Konjunktiv II form of the second verb, on the other hand, would be rettete and would be identical to the past tense. Therefore, we will use Conditional II.