Learn English – Words to refer to “starting level”, “begin level”, “entry level” of knowledge

Infra (below) and supra (above) are also frequently used. You can just put them between brackets after the word you used before.

Other possibilities: aforementioned, above(-)mentioned, previously mentioned, as stated before... Above and earlier are also correct.

Your first sentence is fine. Your second sentence is not.

Begin and start are both verbs, so they can be interchanged.

The equivalent noun of begin is beginning.

The equivalent noun of start is start

(Beginning and start are synonyms and can be interchanged.)

So your second sentence should be:

Why don't you begin from the start?

That is, beginning and start are both "regular" or autonomous nouns. They function the same. They have their own dictionary entry as nouns. The fact that beginning ends in -ing just indicates a different formation. Compare: building. No one doubts that building is a noun!

(Nouns such as beginning and building were evidently made from the verb + -ing centuries ago, but they long since began to be autonomous nouns with all the features of other nouns, such as start (for example, they have plural forms (beginnings, buildings, starts) and they are modified by adjectives. They have their own dictionary entry as nouns.

However, starting is a gerund. It is the -ing form of a verb acting as a noun. As such it has properties of a verb, not a noun. For example it can take a direct object, it is modified by an adverb. It is not a permanent or autonomous noun. Look up starting up in a dictionary: it will redirect you to the verb start. (At least the six dictionaries I tried did this.) This indicates that starting is not considered an autonomous noun (rather, start is).

We don't say Begin at the starting. We say Begin at the starting point Thus we use starting as an adjective. It is also a gerund, as in "Starting the race from the beginning is a good idea." Notice that starting can have a direct object, when it is a gerund.

Now, to slightly complicate matters, since both begin and build are verbs, they can also be used as gerunds: beginning and building. But these words as gerunds can be thought of as different words from the autonomous nouns beginning and building (discussed above) As gerunds, they act the same as the gerund starting:

Beginning (gerund) at the beginning (noun), she read all the letters.

Building (gerund) a large building (noun) takes time.

But we just don't say:

• Starting (gerund) at the starting, she read all the letters.

So starting works as a gerund, but it is not an actual noun, (or a regular noun, or an autonomous noun, or a noun proper--take your pick of terminology).

One can say

In/at the beginning (noun), God created...

but we don't say:

• In/at the starting, God created...

We could say

In/at the start (noun), God created...