Just the way they are processed .. sometimes crushed tomatoes are in a puree and so they're a little thicker and have more "body" than diced. Most canned tomato products are interchangeable in recipes for sauce. Go ahead and used the diced ... it'll be fine. If you have an 8 oz. can of tomato sauce, throw that in, too.
When you add spices or herbs then the flavor will go through teh sauce with crushed but surround with diced. Of course you can crush your own tomatoes. Use whatever tool you have - your feet, a fork, a pestle, a blender.
Sure, no problem, it'll work out just fine. If you want to you can mash them with your tater masher later on.
One thing I recently learned is that if you purchase whole tomatoes and blend them you can make your own crushed later. Or, you can use two knives to dice them up. The whole tomatoes are processed more quickly so they are a tad fresher and tastier. (Chef Michael Smith said this on his show)
Fine to substitute one for the other. Crushed tomatoes are more finely cut up, sort of pureed, that's all, and they tend to be a little thicker, less watery. You can cook the diced tomatoes briefly first and then crush them with a fork before you use them in the recipe, or wait till part way through the cooking of the sauce and do the same. Either way, you'll get pasta sauce and it will taste the same.
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