Learn Hardcore German: Die Lehrerin erklärt, dass Lob im Unterricht eine wichtige Belohnung für unsere Anstrengung ist. - The teacher explains that praise in class is an important reward for our effort.

So die Lehrerin tells you explicitly that the teacher is female. The article die is the nominative singular article for feminine nouns, so die Lehrerin is the subject of the sentence: The (female) teacher explains …

What exactly does erklärt mean here, and how is it different from sagt?

erklärt is the 3rd person singular of erklären, which means “to explain”.

sagen = to say
erklären = to explain (usually more detailed, giving reasons or clarification)

So:

Die Lehrerin sagt, dass … = The teacher says that … (she simply states it)
Die Lehrerin erklärt, dass … = The teacher explains that … (she is clarifying or teaching this idea)

Using erklärt fits well with a teacher in a classroom: she is not just saying something, she is explaining a concept.

What does dass do in this sentence, and why is there a comma before it?

dass is a subordinating conjunction meaning “that” (introducing a clause, not the demonstrative “that one”).

The structure is:

Hauptsatz (main clause): Die Lehrerin erklärt, …
Nebensatz (subordinate clause): dass Lob im Unterricht eine wichtige Belohnung für unsere Anstrengung ist.

In German, you normally put a comma before a dass-clause. The dass-clause is the content of what the teacher explains (indirect/reported speech).

So the pattern is:

[Main clause], dass [subordinate clause with verb at the end].

Why is the verb ist at the very end of the dass-clause?

Because dass is a subordinating conjunction, it sends the finite verb (the conjugated verb) to the end of the clause.

Word order pattern:

Main clause: Verb usually in 2nd position
- Die Lehrerin erklärt, … (verb in position 2)
Subordinate clause with dass: Verb at the end
- … dass Lob im Unterricht eine wichtige Belohnung für unsere Anstrengung ist.

So ist (3rd person singular of sein) must appear at the end of the subordinate clause.

What exactly is im Unterricht? Why im and not in dem Unterricht?

im is a standard contraction in German:

im = in dem

You use in + dem (dative) to talk about being inside or during something:

im Unterricht = in / during (the) class / lessons
im Park = in the park
im Büro = in the office

Unterricht is masculine (der Unterricht = class/lessons), so in the dative singular it becomes dem Unterricht, and then contracts to im Unterricht.

Why is there no article before Lob? Why not das Lob?

Lob is a neuter noun (das Lob) meaning “praise”. It often behaves like an uncountable, abstract noun, similar to “praise” or “water” in English.

In German, when you talk about such things in a general sense, you can omit the article:

Lob ist wichtig. = Praise is important.
Wasser ist lebenswichtig. = Water is essential for life.

If you say das Lob, you are talking about some specific praise:

Das Lob des Lehrers war sehr motivierend.
= The teacher’s praise was very motivating.

In this sentence, we are talking about praise in general in the classroom, so no article is needed: Lob im Unterricht …

In the clause dass Lob im Unterricht eine wichtige Belohnung … ist, which part is the subject: Lob or eine wichtige Belohnung?

Grammatically, Lob is the subject, and eine wichtige Belohnung is a predicate noun/complement.

Structure:

Lob (subject, nominative)
ist (verb)
eine wichtige Belohnung (predicate nominative, also in nominative)

In German, both the subject and the noun after sein (to be) are in the nominative case:

Lob ist eine Belohnung.
Mein Bruder ist Lehrer.
(Both Bruder and Lehrer are nominative.)

The neutral, common order is subject first, so Lob … ist eine wichtige Belohnung … is the standard pattern here.

Why does the adjective end in -e in eine wichtige Belohnung?

This is about adjective endings in German.

Belohnung is feminine (die Belohnung).
eine is the indefinite article, feminine singular, nominative.
After eine (feminine nominative), the adjective takes -e:

Pattern (feminine nominative singular):

eine gute Frau
eine neue Tasche
eine wichtige Belohnung

So wichtige has -e because it describes a feminine, nominative, singular noun with the article eine.

Why is it für unsere Anstrengung and not für unseren Anstrengung or für unsere Anstrengungen?

Several things are happening here:

Preposition “für”:
für always takes the accusative case.
Anstrengung:
- die Anstrengung = the effort, the exertion (feminine noun).
- In the accusative singular, feminine nouns look the same as nominative: die Anstrengung.
unsere:
- Possessive for wir = unser.
- Feminine singular accusative form is unsere.
  So für unsere Anstrengung is correctly accusative.
Singular vs. plural meaning:
In English we often say “our efforts” (plural), but German often uses the singular for abstract nouns like this:
- unsere Anstrengung = our effort / our exertion
  Using the plural unsere Anstrengungen would be possible but shifts the nuance slightly to separate, countable efforts.

So für unsere Anstrengung = for our effort, with unsere and Anstrengung both in the accusative singular, as required by für.

Could we change the word order and say dass im Unterricht Lob eine wichtige Belohnung … ist? Does that change the meaning?

Yes, you can say:

… dass im Unterricht Lob eine wichtige Belohnung für unsere Anstrengung ist.

This is still correct. In German, elements inside a clause can be reordered for emphasis, as long as you respect:

the verb position rules (here: ist must stay at the end of the dass-clause),
and you do not break fixed expressions.

Differences:

dass Lob im Unterricht …
→ Slightly more neutral; focus starts on Lob.
dass im Unterricht Lob …
→ Slightly more emphasis on the setting (in class / during lessons), then comes Lob.

The basic meaning does not change; it is mostly a matter of emphasis and style.

Why is it Belohnung für unsere Anstrengung and not another preposition, like wegen or von?

Belohnung für … is a very common and natural collocation in German:

eine Belohnung für etwas = a reward for something

Examples:

eine Belohnung für gute Noten = a reward for good grades
eine Belohnung für deine Hilfe = a reward for your help

Other prepositions would change the meaning:

Belohnung wegen unserer Anstrengung
→ “reward because of our effort” (more causal/explanatory, sounds less idiomatic here)
Belohnung von unserer Anstrengung
→ would be odd; von doesn’t fit this semantic pattern.

So für is the natural choice to express that the reward is given in return for the effort.

Could we also say Die Lehrerin erklärt, dass Lob im Unterricht eine wichtige Belohnung für unsere Anstrengung sei? What’s the difference between ist and sei here?

Yes, grammatically you can say:

Die Lehrerin erklärt, dass Lob im Unterricht eine wichtige Belohnung für unsere Anstrengung sei.

Here, sei is Konjunktiv I (subjunctive I) of sein and is traditionally used in reported speech to show that this is someone else’s statement, not the speaker’s own claim.

Differences in usage:

… dass … ist. (Indicative)
→ Most common in everyday spoken German. Neutral, straightforward.
… dass … sei. (Subjunctive I)
→ More common in formal written German, news reports, academic or very careful style.
→ Slightly distances the narrator from the content, focusing on reporting what someone said.

In normal spoken language, ist is far more typical and natural in this sentence.