phrase usage - When can I remove the word "that" in a sentence?

I have this bad writing habit, I use the word 'that' so many times. I know it is grammatically valid, but using 'that' too often is not good. But I don't know in which sentences I can remove the word 'that'.

I'm more concerned with spoken english, not written English. So in spoken, I think it doesn't have to be perfectly correct and sometimes 'that' can be removed without altering the meaning of the sentence. Can you tell me when 'that' can be removed? I'm looking for people to point out a few sentences where 'that' can be removed and a explanation of why the removing is necessary.

I believe 'that' can be removed in all these sentences without changing the meaning.

Another question I have, whether these 'that' are must based where it is used? I mean if I use these sentences in a official letter without "that", is that acceptable? If use these sentences in a story without "that", is that acceptable?

1. 
   

   It's the same girl that we saw in the family photo

   
   



2. 
   

   Alex understands that he's protected by his family.

   
   


3. 
   

   I came to know that you got stuck in the traffic.

   
   


4. 
   

   Many people started saying that they saw ghost.

   
   


5. 
   
   

   Okay, the reason I'm helping you is that I need help from you.

   
   

Answer

In sentence 1, that acts as a relativizer (relative pronoun). It may be dropped (in any register) because it acts as the direct object of the verb in the relative clause. It could not be dropped in formal English (though it often is in informal spoken English) if it were the subject of the verb in the relative clause:

^✲ It's the same girl Ø took our family photo.

In the remaining sentences, that acts as a subordinator (subordinating conjunction). In sentences 2, 3 and 4, that may be dropped because the subordinate clause which it heads is the direct object of the verb in the main clause and is in its ordinary position immediately after that verb.

If that played another role, such as subject, or if the subordinate clause were displaced to another position, that could not be dropped, because it would not be clear that it is in fact a subordinate clause:

^✲ Ø he's protected by his family is understood by Alex ... The clause falls at the beginning of the sentence, before the verb is, because it has become the subject. That cannot be dropped.
^✲ I came to know some eight or ten days after I got the report Ø you got stuck in traffic. ... Here the subordinate clause has been separated from its governing verb by a fairly long ('heavy') adverbial phrase . You could probably get away with dropping the that in speech, but it cannot be dropped in formal writing.

In sentence 5, and in these rewrites of sentences 4 and 5, the situation is a little different: These subordinate clauses are predicative complements of BE, and in speech that may be dropped even if the clause is moved to the front. In writing it's permitted, but not advisable; you really want to give the reader as many clues to your structure as possible:

^? What many people are saying is Ø they saw a ghost.
^? Ø I need help from you is the reason I'm helping you.

In other uses, as a demonstrative adjective or a demonstrative pronoun, that may not be dropped.

I want that puppy. but not ^✲ I want puppy.
John took that from Shakespeare. but not ^✲ John took from Shakespeare.

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^✲ marks an utterance as unacceptable
^? marks an utterance as possibly unacceptable
Ø marks the place where that is omitted