Something is simple if it doesn't have a lot of elaborate parts, or is very easy to understand. The picture is of a very simple flower arrangement.
The opposite of simple might be elaborate, complicated or difficult.
Single is used to describe just one of something or someone. For example, if you're not in a relationship, you can say you're single.
If you are sinister you are frightening, threatening or intimidating.
The house in the picture is sinister.
Something is skinny if it is very thin, even to the point of being unhealthy.
Someone is slender if they are thin and graceful. Young women are often slender.
Something is slippery if it very hard to get a hold of, or you can't stick to it. A slippery surface is one your shoes or feet don't grip onto, so you fall over when you are walking on them. Slippery things are usually very smooth and are often wet and slimey.
Some things that are slippery are ice, wet roads, banana peels, and rocks at the beach at low tide that have algae growing on them.
The opposite of slippery is sticky.