Learn Hardcore German: Der Lärm beeinflusst meine Arbeit, deshalb schließe ich das Fenster und beobachte die Straße. - The noise influences my work, therefore I close the window and observe the street.

German main clauses are verb-second (V2). If you put a sentence adverb like deshalb in first position, the finite verb must come next, and the subject follows: Deshalb (1st position) schließe (2nd = verb) ich (subject) das Fenster. Without deshalb, you’d have the neutral order Ich schließe das Fenster.

Is the comma before deshalb correct? Could I use a semicolon or a period instead?

Yes, the comma is correct. You’re separating two independent main clauses, and deshalb is a conjunctive adverb, not a conjunction. A semicolon or a period would also be fine:

Der Lärm beeinflusst meine Arbeit; deshalb schließe ich das Fenster …
Der Lärm beeinflusst meine Arbeit. Deshalb schließe ich das Fenster …

Can I use weil or denn instead of deshalb? What changes?

Yes, but the structure changes because weil and denn mean “because,” not “therefore.” To keep the same meaning, you must flip cause and effect:

With weil (subordinating, verb to the end): Weil der Lärm meine Arbeit beeinflusst, schließe ich das Fenster …
With denn (coordinating, no inversion): Ich schließe das Fenster, denn der Lärm beeinflusst meine Arbeit. You can’t just replace deshalb in place with denn/weil; you must also reorder the clauses.

Why is there no preposition like auf after beeinflusst?

Because beeinflussen is a transitive verb that takes a direct object in the accusative: etw./jdn. beeinflussen. So you say Der Lärm beeinflusst meine Arbeit, not ✗beeinflusst auf meine Arbeit. If you want a prepositional phrase, use the noun phrase Einfluss auf: Der Lärm hat Einfluss auf meine Arbeit.

Which cases and articles are used here, and why?

Der Lärm: nominative (subject), masculine.
meine Arbeit: accusative (direct object of beeinflusst), feminine; possessive meine agrees with feminine accusative.
das Fenster: accusative (direct object of schließe), neuter; article stays das in acc. sing.
die Straße: accusative (direct object of beobachte), feminine; article stays die in acc. sing.

Why is it meine Arbeit and not meinen Arbeit?

Because Arbeit is feminine. In the accusative singular:

Masculine takes -en (e.g., meinen Hund).
Feminine stays -e (e.g., meine Arbeit).
Neuter stays the base form (e.g., mein Fenster).

Why can the second clause omit ich: … und beobachte die Straße?

This is a standard coordination pattern called “gapping.” When two coordinated clauses share the same subject, German often omits the repeated subject in the second clause:

(Ich) schließe das Fenster und (ich) beobachte die Straße. The omitted subject is understood to be the same ich.

Would und ich beobachte die Straße be better? What about und beobachte ich die Straße?

und ich beobachte die Straße is perfectly fine and is stylistically the safest in writing.
und beobachte die Straße (subject omitted) is also fine and very common.
und beobachte ich die Straße as a statement sounds odd; it’s typically used in questions or special emphasis. Prefer one of the first two.

Why Straße has ß, schließe has ß, but beeinflusst has ss?

Modern German orthography:

ß follows a long vowel or diphthong: Straße (long a: aː), schließe (ie = long i).
ss follows a short vowel: beeinflusst (short u before ss). Note: In Swiss Standard German, ß is not used, so you’ll see Strasse, schliesse.

Is beeinflusst the best verb for noise affecting work? Could I use stört or beeinträchtigt?

beeinflussen = to influence/affect (neutral about positive/negative).
stören = to disturb/bother; very idiomatic for noise: Der Lärm stört mich bei der Arbeit.
beeinträchtigen = to impair (stronger negative impact): Der Lärm beeinträchtigt meine Arbeit. All are correct; choose based on nuance. Another option: Der Lärm hat Einfluss auf meine Arbeit.

What’s the difference between schließen and zumachen for the window?

They’re near-synonyms:

schließen is neutral/formal: Ich schließe das Fenster.
zumachen is more colloquial and separable: Ich mache das Fenster zu. Both are widely used.

Can deshalb appear elsewhere in the clause?

Yes. Common placements:

At the start (strong link to previous sentence): Deshalb schließe ich das Fenster …
In the midfield after the subject: Ich schließe deshalb das Fenster … Front position is most typical when connecting two main clauses.

What time reference does the present tense convey here?

German present (Präsens) can express:

An action happening now: “I’m closing the window now …”
A near-future plan: “I’m about to close the window …” Context decides. Here it most naturally means “now.”

Why are words like Lärm, Arbeit, Fenster, Straße capitalized?

All nouns are capitalized in German, regardless of position in the sentence.

Does beobachten need a preposition, or can it take a direct object?

beobachten takes a direct object in the accusative: die Straße beobachten. If you mean “look at,” you might also hear:

(sich) etwas ansehen: Ich sehe mir die Straße an.
auf etwas schauen/gucken: Ich schaue auf die Straße. But beobachten is perfectly correct and means “observe/watch (closely).”