Learn English – “It is done” what is the sentence structure

First, let's look at a similar sentence:

1a. [ All I wanted ]　was 〔an ice cream cone〕.
1b. [ The only thing I wanted ]　was 〔an ice cream cone〕.

In example 1a, all I wanted is a noun phrase.
It means the only thing I wanted, so example 1b means the same thing.

Your example is similar:

​2. [ All I did ]　was 〔hand someone a bag〕.

Here, the bare infinitival clause hand someone a bag is used as a complement of specifying be.
This is one of the few functions of bare infinitival clauses—they appear mainly as complements of a few specific verbs, and one of those verbs is be.

If you'd like, you can call it a nominal bare infinitive clause, because it appears where a noun phrase would normally be expected. But in any case, this construction is grammatical and totally normal.

Broadly, you can consider them as synonyms. This is to make us understand that when you are talking about something that happened in 'past', grammatically you are talking in 'past tense'. And, it is simply past tense without describing any mood or completion of a particular event (or else it then starts having other types of past i.e. past perfect etc.).

Wikipedia says...

The past tense is a grammatical tense whose principal function is to place an action or situation in past time. In languages which have a past tense, it thus provides a grammatical means of indicating that the event being referred to took place in the past. Examples of verbs in the past tense include the English verbs sang, went and was.

Having this said, past tense is actually an umbrella term which covers many things in English grammar. Under past tense, there could be many other types such as past perfect, past participle and so on.

Past tense is generally considered as a Simple Past if you have not specified anything about it.

About.com describes it as a synonym by putting it into brackets. There are also other terms for this mentioned on the same page.

The simple past tense (also known as the past simple or preterite) of regular verbs is marked by the ending -d, -ed, or -t. Irregular verbs have a variety of endings. The simple past is not accompanied by helping verbs.