italki is "a" great instument... "a" means one, as in: one example of a great instrument one of many great instruments Unless you are trying to say "Italki is the greatest (best) instrument for learning english." Oddly enough, "tool" might sound better because "instrument" sounds too much like a tangible object or piece of equipment. You could also refer to it as a "website" or "language exchange" or something. "Our house was ignited by lightning." "Our house burned down from a lightning strike." More natural than "ignited by" would be: "Our house was set on fire by lightning." or "...hit/struck by lightning and set on fire." Third sentence is correct but pretty vague. Don't know the details behind what you are trying to convey, so can't improve upon it.

1. Italki is a great platform for learning English. 2. Lightning caused our house to catch fire. 3. totally fine +1

1. Italki is a great instrument for learning English. (The usage of instrument here is correct, but I find it kind of awkward and I would prefer to use the word "tool".) 2. Our house was ignited by lightning. 3. totally fine