Trigger Context Variables

trigger AccountTrigger on Account (before update) {
    for (Account acc : Trigger.new) {
        System.debug('New value: ' + acc.Industry);
    }

    for (Account acc : Trigger.old) {
        System.debug('Old value: ' + acc.Industry);
    }
}

1. trigger AccountTrigger on Account (before update) { — the trigger.
2. for (Account acc : Trigger.new) { — Trigger.new is a List<Account> (Module 15) holding the records as they will be saved.
3. System.debug('New value: ' + acc.Industry); — logs the incoming value.
4. } — closes the loop.
5. (blank line)
6. for (Account acc : Trigger.old) { — Trigger.old holds the records as they were before this change — available on update and delete, since insert has no "before" state to compare against.
7. System.debug('Old value: ' + acc.Industry); — logs the prior value.
8. } — closes the loop.
9. } — closes the trigger.

trigger AccountTrigger on Account (before insert, before update) {
    if (Trigger.isInsert) {
        System.debug('This is an insert.');
    }
    if (Trigger.isUpdate) {
        System.debug('This is an update.');
    }
    if (Trigger.isBefore) {
        System.debug('This is running before the save.');
    }
}

1. trigger AccountTrigger on Account (before insert, before update) { — a single trigger can handle multiple events at once, as declared in the parentheses.
2. if (Trigger.isInsert) { — checks which event is currently firing.
3. System.debug('This is an insert.'); — runs only for inserts.
4. } — closes the if.
5. if (Trigger.isUpdate) { — a different check.
6. System.debug('This is an update.'); — runs only for updates.
7. } — closes the if.
8. if (Trigger.isBefore) { — checks the timing.
9. System.debug('This is running before the save.'); — Trigger.isInsert, Trigger.isUpdate, Trigger.isBefore, Trigger.isAfter, and their counterparts let the code branch (Module 6) based on exactly which event is currently firing.
10. } — closes the if.
11. } — closes the trigger.

trigger AccountTrigger on Account (before update) {
    for (Account acc : Trigger.new) {
        Account oldVersion = Trigger.oldMap.get(acc.Id);
        if (acc.Industry != oldVersion.Industry) {
            System.debug('Industry changed from ' + oldVersion.Industry + ' to ' + acc.Industry);
        }
    }
}

1. trigger AccountTrigger on Account (before update) { — the trigger.
2. for (Account acc : Trigger.new) { — loops over the incoming records.
3. Account oldVersion = Trigger.oldMap.get(acc.Id); — Trigger.oldMap is exactly Module 18's Map<Id, sObject> pattern, pre-built by Salesforce — looking up a specific record's before version by Id, rather than by matching list position.
4. if (acc.Industry != oldVersion.Industry) { — compares the new value to the specific matching old value.
5. System.debug('Industry changed from ' + oldVersion.Industry + ' to ' + acc.Industry); — this reliability is what makes comparing "did this specific field actually change?" trustworthy.
6. } — closes the if.
7. } — closes the loop.
8. } — closes the trigger.