Learn Hardcore Italian: Metto il sacco di pane sul tavolo. - I put the sack of bread on the table.

In Italian the subject pronoun (io, tu, lui, etc.) is usually dropped because the verb ending already shows who is doing the action. The -o ending on Metto tells you the subject is “I,” so you don’t need io.

How do I replace il sacco di pane with a pronoun?

Since il sacco di pane is a masculine singular direct object, you use the pronoun lo. In Italian the object pronoun comes before the verb:
Lo metto sul tavolo.

Why is there an il before sacco, but no article before pane?

sacco is a countable singular noun, so you need an article (il sacco = “the bag”).
pane here is a mass noun (bread as a substance), and when you indicate its content with di in a generic sense, you omit the article.

Why is it di pane and not del pane?

di pane (with no article) expresses a generic content (“bag of bread”).
del pane (which is di + il pane) would be “of the bread,” implying a specific bread already known or mentioned.

What does sul stand for, and could I say su il?

Sul is the contraction of su + il, meaning “on the.” In standard Italian you must contract su with the definite article (su + il = sul), so su il would be incorrect.

Why sul tavolo and not al tavolo or a tavola?

sul tavolo = “on the table” (physically placed on top).
al tavolo = “at the table” (beside or near it, e.g. you sit at a table).
a tavola is an idiomatic way to say “at the dining table” (commonly used when you sit down to eat).

Can I say Sul tavolo metto il sacco di pane instead?

Yes. Italian word order is quite flexible. Fronting Sul tavolo puts emphasis on the location. The meaning stays the same.