だけで

だけで means "just by doing" or "with just": that one small thing is enough to bring about the result. It follows the dictionary form of a verb, or a noun.

The result is usually more than you'd expect from so little: a strong feeling, a big effect, an easy success.

After a た-form

When the small action has already happened, the verb goes into the た-form.

だけで and だけでは

With は added, だけでは is followed by a negative: that one thing is not enough. だけで says what suffices; だけでは says what doesn't.

だけでなく, "not only", is a different pattern that adds to the list rather than stopping at one item.