I want to know how to write test classes to execute  the trigger code coverage? how should we write on what basis? for the trigger i have writtened some test classes but it wasnt giving code coverage for me. I almost executed all lines in trigger. where i am behind?

trigger createTaskForEmail on Task (before insert) {
   for (Task t : trigger.new) {
               if (t.status == 'Completed' && t.subject.startswith('Email:')) {
                          t.status = 'Completed';
                          t.description = t.description.substringAfter('Body:');
                          t.Activity_Direction__c = 'Email';
                          t.Activity_type__c='Email';
                }
     }
}

For this trigger I have writtened Test class some thing like below which gives code coverage up to 33 % only? what else i have left?

@isTest(SeeAllData = false)
private class Test_createTaskForEmail
{
static testMethod void UnitTestForException1()
{

  
Task ta = new task(Activity_type__c='Email'  , Activity_Direction__c='Email',status = 'completed');
insert ta ;
update ta ;

}
}

Like that i have other trigger i.e

trigger UpdateEventRating on Event (before insert, before update) {
for(Event E : Trigger.new){
if(E.Rating__c=='-Excellent Meeting (committed to business within 3 weeks)'){
E.Rating_Value__c=5;
}
else
if(E.Rating__c=='-Good Meeting (Resulted in client meeting agreed to/set or booked a seminar)'){
E.Rating_Value__c=4;
}
else
if(E.Rating__c=='-Productive Meeting (Client specific illustration requested or next meeting scheduled)'){
E.Rating_Value__c=3;
}
else
if(E.Rating__c=='-Average Meeting (Agreed to continued discussions, no client specific activity)'){
E.Rating_Value__c=2;
}
else
if(E.Rating__c=='-Poor Meeting (No potential for future business, no further activity needed)'){
E.Rating_Value__c=1;
}
}
}

Test Class For this trigger is this gives 53% code coverage only

@isTest(SeeAllData = false)

private class Test_UpdateEventRating 
{
static testMethod void UnitTestForException()
{
test.startTest();
Event e = new Event(ActivityDate = system.today(), ActivityDateTime = system.now(), DurationInMinutes = 2);
insert e ;
update e ;
test.stopTest();
}
}