None of these are really idiomatic at all. They all state clearly what they mean to say. 1. Both are correct. 2. Both are correct, but a slight spelling error with "every" in the first part. 3. "Mark the icon with a triangular" - triangular is incorrect in all of these. Triangular is an adjective describing the shape; not a verb or a noun. The noun form is triangle. colloquially, I suppose you could turn "triangle" into a verb, but this is not common or technically "correct." If you correct number 3, I'd say that "Mark the icon with a triangle" would be the best option. Hope this helps! Best of luck to you!

"Let's go out for dinner" is probably the better option. "I play baseball every day" is better. And I think what you're trying to say is "Mark the icon with a triangle" (triangular is an adjective describing a shape), or "Put a triangle on the icon" The other options don't really make sense...

In the first sentence, both are correct and the second expression is more idiomatic. In the second sentence, the second one is wrong. I'm not sure if you meant to type "I go play baseball every day." If that is the case the second one would be the more idiomatic expression. The third one is all wrong.