Stardew Valley tasks you with rebuilding and revitalizing your grandpa's old farm, and that requires a ton of building materials. One of the building materials you will need is Hardwood, which is much less common than regular wood. There is a reliable way to get Hardwood, but you're going to need to upgrade your axe first.
How to get Hardwood
Hardwood is obtained by destroying wooden stumps and logs with an axe. To destroy stumps you will need to upgrade to a Copper Axe and to destroy logs you will need a Steel Axe. Wooden stumps and logs can be found in a few different places around town, like the Mountains and Cindersap Forest. There will also be several of both around your farm that need to be destroyed as part of the clean up process.
The best place to get Hardwood is from the Secret Woods, where six stumps will spawn every day. The Secret Woods are found in the northwest corner of the Cindersap Forest. From your farm head directly south to enter the Cindersap Forest. You will enter on the side of Marnie's Farm. Head west to the upper left corner of the forest. This area is initially blocked by a wooden log, so you will need at least a Steel Axe to enter. If you choose either the Forest Farm or Four Corners Farm, both farm types have daily respawning stumps, providing even more Hardwood.