この is used when you want to directly modify a noun: この方法 = this method.
これ stands by itself and means this one/this thing.
So:
You cannot say これ方法.
Yes—確実 is what learners usually call a na-adjective. In grammar terms, it behaves differently from an i-adjective.
That is why it works like this:
Before a noun, it takes な. At the end of a sentence, it can be followed by です in polite speech.
So 確実 is not an -i adjective like 高い or 安い.
Because 確実 is not an i-adjective. Only true i-adjectives end in い as part of their grammar.
For example:
With 確実, the correct forms are:
Yes, but the nuance changes.
この方法は確実です means as for this method, it is reliable/certain. It sounds like a general statement about the method.
この方法が確実です often sounds more like this method is the reliable one, especially when choosing among alternatives or answering a question like Which method is the sure one?
So both are possible, but は and が give different emphasis.
確実 often means something like certain, reliable, sure, or surefire, depending on context.
In a sentence like this, it usually suggests that the method is dependable or likely to work. It does not necessarily mean absolutely guaranteed in every situation, but it gives a strong sense of confidence.
So the exact English wording can vary with context, even though the Japanese grammar stays the same.
です mainly marks the sentence as polite. In this sentence, it also gives the standard polite non-past form.
Japanese non-past can cover:
So この方法は確実です can mean that the method is reliable, or more generally that it will be reliable in use, depending on the situation.
The plain form is:
この方法は確実だ。
In conversation, people may also soften or shorten things depending on tone, but 確実だ is the basic plain counterpart of 確実です.
So:
No. Standard Japanese is normally written without spaces:
この方法は確実です。
Spaces are often added in beginner materials to make the sentence easier to read. In natural writing, you usually will not see them.