Shell Programming and Scripting

I am trying to write a simple if statement but that driving me crazy with syntactical erorrs.

This is what I managed to come up

Code:

 
#!/bin/ksh
USERNAME=`whoami`
if [ "${USERNAME}" = "report" ];then
echo " you have logged in as report user"
elif ["${USERNAME}" = "extract" ]; then
echo " you have logged in as extract user"
else " I dont recognize you"
fi

can one of you explain me
1. where to use spaces and where not to?
2. some time I see double square brackets used [[ ...]] when should I use these?
3.should there be a space between variable and value in condition like
<variable><space><operator><space><value>
4.when should i use either single/double quotes around variables and values?
5. should 'then' be used after every conditinal statement ?

The code above it still gives me error

Code:

./trial.sh[9]: [extbatch:  not found.
./trial.sh[11]:  I dont recognize you:  not found.

I am using AIX and ksh. Not sure if it makes any difference

Code:

 
#!/bin/ksh
USERNAME=`whoami`
if [ "${USERNAME}" = "report" ] ; then
   echo " you have logged in as report user"
elif [ "${USERNAME}" = "extract" ] ; then
   echo " you have logged in as extract user"
else 
  echo " I dont recognize you"
fi

@dr.House . Thank you. I now understand why ppl say. you need a fresh pair of eyes looking into your code.

I had few questions about if syntax in my post. I would be thankful if you can advice me

Some very general answers: -

1) Use spaces as you show in question 3, not during assignments such as X=12.
2) I think there are two versions of ksh in general use, 88 and 93? [[ ]] is ksh 88 whereas [ ] is ksh 93 Most commercial UNIX environments still use 88 somewhat inexplicably. The company I work for is only 5 years old but uses 88!!
3) Yes for conditional statements.
4) Big subject. Quoting is one of the trickiest subjects in shell scripting. On a simple level if a string contains spaces you probably want to quote it but not always.
5) In 88 at least you want to put a "then" after an if although if statements are not the only types of conditional statements,

Experiment