Can a Wild Shaped druid concentrate on a spell they cast before transforming, and can they cast new spells while transformed?

Yes, you can hold concentration on a spell!

When you use Wild Shape, transforming doesn't break your Concentration or otherwise interfere with a spell you've already cast D&D 5e SRD 5.2 — Pla.... Any ongoing effects on you, including spells, simply carry over from your normal form to your Beast form D&D 5e SRD 5.2 — Gam....

However, casting new spells is a different story:

• Normally: You cannot cast new spells while shape-shifted D&D 5e SRD 5.2 — Pla....
• At Level 18: Druids gain the Beast Spells feature. This allows you to cast spells while in your Beast form, with one restriction: you can't cast any spell that requires a Material component with a specified cost, or one that consumes its Material component D&D 5e SRD 5.2 — Pla....

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