Legno vs. legna
While both legno and legna can be translated as "wood" in English, they are not interchangeable in Italian. The key difference lies in the purpose and form of the wood.
Legno
A2Legno is a masculine noun that refers to wood as a material, the substance from trees used for construction, furniture, crafts, and art.
Questo tavolo è fatto di legno di noce.
(This table is made of walnut wood.)
Mi piace l'odore del legno appena tagliato.
(I like the smell of freshly cut wood.)
Il falegname sta lavorando un pezzo di legno.
(The carpenter is working on a piece of wood.)
La casa ha un bellissimo pavimento in legno.
(The house has a beautiful wooden floor.)
Le matite sono generalmente fatte di legno di cedro.
(Pencils are generally made of cedar wood.)
La scultura è stata intagliata in un blocco di legno massiccio.
(The sculpture was carved from a solid block of wood.)
Legna
B1Legna is a feminine collective noun that specifically means firewood, or wood that has been cut and prepared for burning in a fireplace, stove, or oven.
Dobbiamo comprare più legna per l'inverno.
(We need to buy more firewood for the winter.)
Ho passato il pomeriggio a tagliare la legna.
(I spent the afternoon chopping the firewood.)
Metti un altro pezzo di legna sul fuoco, per favore.
(Put another piece of firewood on the fire, please.)
La pizza cotta nel forno a legna è la migliore.
(Pizza cooked in a wood-fired oven is the best.)
C'è una grande catasta di legna dietro la casa.
(There is a large pile of firewood behind the house.)
La stufa a legna riscalda tutta la stanza.
(The wood-burning stove heats the whole room.)
Summary
In essence, the difference is simple: legno is the material you use to build or create things (wood), while legna is the fuel you use to burn for heat or cooking (firewood). If you can make a chair with it, it's legno. If you put it in a fireplace, it's legna.