Tricky Punctuation - Comma
The differencesbetween Czech/Slovak and English punctuation are not many. Most of them do notchange the meaning of a sentence; however, using the comma is sometimes reallytricky. One main difference using a comma is that there is NO COMMA before THATin English. Czech and Slovak learners tend to put a comma there, as in both ofthese languages they do write commas before ‘že’ (translated “that” in English)or ‘ktorý, který’ (other words which can be translated with “which” OR “that”).
Rule 1
Don’t put a comma before ‘that’ in reported speech.
Rule 2
Relative clauses describe or define the words right before them. They usually start with which, who, whom, or that. Sometimes they require commas and sometimes they do not. You must decide whether the clause is a defining relative clause (the identity of the words in the sentence is not clear without the clause – it is necessary) or a non-defining one (the relative clause just comments on the main clause – it is not necessary for understanding).
In defining relative clauses there are no commas (you need the clause, so you can’t omit it — don’t separate it with a comma) and ‘that’ can be used. In non-defining relative clauses ‘that’ can’t be used.
Example of a defining relative clause:
My son that/whois in Poitier now sent me a postcard last week . (He is not my only son, so I need to identify the one who sent me a postcard. No commas make it clear that the information is needed to identify which son.)
Example of a non-defining relative clause:
My husband, who is in Poitier now, sent me a postcard last week . (Is it clear that I have just one husband 
?)
Rule 3
Although allowed in Czech and Slovak, two independent clauses cannot be connected with only a comma in English. They must also have a connecting word, such as and, but, so, and so on. Or they must be made into two sentences. Examples:
Incorrect: My best friend was involved in a car accident yesterday, she went to the hospital with multiple injuries.
Correct: My best friend was involved in a car accident yesterday. She went to the hospital with multiple injuries.
or
Correct: My best friend was involved in a car accident yesterday, so she went to the hospital with multiple injuries.