- That he is very sick, is evident.
The presence of that comma in there, where it separates the subject from the rest of the clause, is an indication that an old style of punctuation is being used. Several hundred years ago, it was acceptable to use a comma like that; but nowadays, that style of punctuation is considered to be unacceptable.
Nowadays, one way of parsing the sentence:
- That he is very sick is evident.
is to consider that the subject of the main clause is a subordinate clause in the form of a declarative content clause. In the below, the subject is in brackets and the main verb (or head verb) of the clause is bolded:
- [That he is very sick] is evident.
The subordinate clause:
can be parsed as having the subject "he", the head verb "is", and the predicative complement as the adjective phrase "very sick". The word "that" is a marker of clausal subordination. That marker is usually obligatory when a declarative content clause is functioning as the subject of a main clause.
Note that, in general, for your type of example, which has a content clause as subject, there is an extraposition construction as an alternative: It is evident that he is very sick. There are less pragmatic constraints on the extraposed version. And so, usually the extraposed version is preferable to the non-extraposed version.
Hopefully this is enough for you to get a sense of how your example could be parsed in a modern grammar.