have a (good/solid/sound/etc.) grasp of/on (something)
To have a firm, clear understanding or determination of something. I hope everyone has a solid grasp of the material, as you'll be tested on it next week. I thought I had a pretty good grasp on English idioms, but I don't understand this one at all!
get a (good/solid/sound/etc.) grasp of/on (something)
To attain a firm, clear understanding or determination of something. You will need to get a solid grasp of the material, as you'll all be tested on it next week. I'm trying to get a grasp on English, but I don't understand some of these idioms at all!
be (as) solid as a rock
To be stable, sturdy, and reliable. They've been married for 50 years, so their relationship has to be solid as a rock. That table must be as solid as a rock to withstand the ceiling collapse with hardly a scratch.