You're giving a presentation about the sales results of a product which your company recently started selling. The company has been selling more and more of the product each week. You say:
Sales have been steadily increasing since the launch.
"Sales" are the number of items you sell or the amount of money you make from selling things. Sales can be "strong" or "weak", and they can "increase" and "decrease", or "rise" and "fall".
To "steadily increase" means to keep increasing by the same amount, again and again. Here's an example of some steadily increasing numbers:
20 > 25 > 30 > 36 > 41 > 45
Here are some numbers that are increasing, but not steadily:
20 > 29 > 27 > 28 > 40 > 40
This is how to talk about an action that started in the past, and is still happening now.
Whenever you use the word "since", you have to use the perfect form "have done":
I've known him since he was born.
We've been ranked #1 every year since 1990.
You shouldn't use the simple past tense ("knew", "were") with "since".
When a rocket "launches", it shoots up into the air. But to "launch" a new product or business means to create it and release it to customers. When you're talking about this event, you call it "a launch".
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