Learn Hardcore Italian: Questa sera, con quel telescopio, guarderemo la Luna insieme dal portico. - This evening, with that telescope, we will look at the Moon together from the porch.

Questa sera literally means “this evening” and is slightly more formal or emphatic. Stasera is its contracted, more colloquial form. In everyday speech you’ll hear stasera most often, but both are interchangeable.

Are the commas around con quel telescopio necessary?

No, they’re not strictly required. In Italian, commas can set off parenthetical or explanatory phrases. Here con quel telescopio is an extra detail inserted into the sentence. You could write:
Questa sera con quel telescopio guarderemo la Luna insieme dal portico.
The commas simply make the rhythm clearer.

What role does con play in con quel telescopio? Could we use another verb?

Con means “with” in the sense of using an instrument. It tells you with what you’ll watch the Moon. You could also say usando quel telescopio (“using that telescope”), but con + instrument is more natural for expressing the means.

Why is quel used before telescopio? What’s the difference between quel, quello and quell’?

These are all masculine singular demonstratives, chosen based on the following word’s initial letter:

quel before most consonants: quel telescopio
quello before s+consonant, z, gn, ps: quello studente, quello zaino
quell’ before vowels: quell’amico
They all translate as “that.”

How is the future tense guarderemo formed, and why use it here?

Guarderemo is the 1st-person-plural future of guardare. Formation for –are verbs:

Drop the final -e from the infinitive.
Add future endings: -erò, -erai, -erà, -eremo, -erete, -eranno.
We choose the future because the action (“we will watch”) is set in the future (tonight).

Why do we say la Luna with an article, and why is it capitalized?

In Italian, celestial bodies usually take the definite article: il Sole, la Luna, Venere, etc. When referring to the actual Moon, many writers also capitalize Luna like a proper noun. You cannot drop the article in standard usage.

Is insieme fixed in this position? Could we move it?

The adverb insieme (“together”) is quite flexible. Common placements include:

After the verb: guarderemo insieme la Luna
At the end: guarderemo la Luna insieme
At the beginning (for emphasis): Insieme, guarderemo la Luna
All are grammatically correct; the meaning stays “we will do it together,” though emphasis shifts.

What does dal in dal portico stand for, and why not su or nel?

Dal = da + il, literally “from the.”

dal portico = “from the porch” (viewpoint or vantage point)
su portico would mean “on the porch”
nel portico means “inside the porch”
Using dal emphasizes from where you’re observing the Moon.