Manx,
What kind of grass do you have? Is it a cool weather or hot weather grass? Is it in the shade or sun? Also, have you made sure you do not have pests destroying your grass?

As for cutting it, you should never cut more than 1/3 of the total grass height for healthy lawn.

You might try raking up the brown spots to loosen the soil, putting down a good starter fertilizer, then put some seed down, lightly cover the seed with some wheat straw and keep it watered. The spots will fill in(at least for now depending on what kind of grass it is).

Hope this helps

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