Mi ne pensas, ke ni devas ĉiam ŝanĝi niajn kutimojn, sed foje nova kutimo estas tre necesa.

Ke introduces a subordinate clause, like English that.

So in Mi ne pensas, ke ni devas..., the part after ke is the thing being thought about.

A very important point for English speakers: Esperanto usually keeps ke where English often drops that. So even if English might say I don’t think we need..., Esperanto normally still uses ke: Mi ne pensas, ke...

These mean different things because ne changes what is being negated.

• Mi ne pensas, ke ni devas... = I do not think that we must...
• Mi pensas, ke ni ne devas... = I think that we must not...

In Esperanto, ne usually goes directly before the part it negates.

So here, ne is negating pensas, not devas.

Because devi is a modal verb, and modal verbs are followed by an infinitive.

• devas = must / have to
• ŝanĝi = to change

So ni devas ŝanĝi means we must change.

This works much like English:

• we must go
• ni devas iri

The second verb stays in the infinitive form ending in -i.

The -i ending is the infinitive ending in Esperanto, equivalent to English to change, to go, to do, and so on.

After devas, you use the infinitive:

• devas ŝanĝi
• devas lerni
• devas fari

You do not conjugate the second verb:

• correct: ni devas ŝanĝi
• not: ni devas ŝanĝas

Ĉiam is an adverb meaning always, and Esperanto adverbs are fairly flexible in position.

Here, ni devas ĉiam ŝanĝi... is natural and means that the idea of always applies to the action.

You could also hear:

• ni ĉiam devas ŝanĝi...

That is also grammatical, but the emphasis is slightly different. Esperanto word order is often chosen for style or emphasis, not because only one position is possible.

Because kutimojn is:

• plural: -j
• accusative: -n

So:

• kutimo = habit
• kutimoj = habits
• kutimojn = habits as a direct object

And niajn agrees with kutimojn.

In Esperanto, adjectives and adjective-like words agree with the noun in number and case. Since nia behaves like an adjective, it also becomes:

• nia = our
• niaj = our, plural
• niajn = our, plural accusative

So:

• niajn kutimojn = our habits, as the object of ŝanĝi

Yes. The possessive forms built from personal pronouns behave like adjectives:

• mia = my
• via = your
• lia = his
• ŝia = her
• ĝia = its
• nia = our
• ilia = their

Because they act like adjectives, they can take -j and -n to agree with the noun:

• nia kutimo
• niaj kutimoj
• niajn kutimojn

This is very regular in Esperanto.

Foje means sometimes, at times, or on occasion.

It comes from fojo, meaning an occasion or a time in the sense of an occurrence.

So:

• fojo = an occasion / a time
• foje = sometimes / occasionally

It is an adverb, which is why it ends in -e.

Because nova kutimo is not the direct object of a verb. It is the complement of estas.

In Esperanto, after esti, you normally use the basic form, not the accusative.

So:

• nova kutimo estas tre necesa = a new habit is very necessary

Here:

• kutimo is the subject
• nova describes kutimo
• necesa also describes it

There is no direct object, so no -n.

Because the second clause is making a general statement: sometimes a single new habit is very necessary.

It does not need to match the number of the earlier noun. The sentence first talks about our habits in general, then says that sometimes one new habit can be very necessary.

So the shift from plural to singular is perfectly normal.

Tre is an adverb meaning very, and necesa is an adjective meaning necessary.

Adverbs in -e often modify adjectives, just like in English:

• tre bona = very good
• tre grava = very important
• tre necesa = very necessary

So tre modifies necesa, and necesa describes nova kutimo.

Esperanto punctuation is often a bit more regular than English punctuation.

• A comma before ke is common because it introduces a subordinate clause.
• A comma before sed is also standard, because sed is a coordinating conjunction meaning but.

So this punctuation is normal and helps show the structure clearly:

• main clause: Mi ne pensas
• subordinate clause: ke ni devas ĉiam ŝanĝi niajn kutimojn
• contrasting clause: sed foje nova kutimo estas tre necesa

Not without changing the meaning.

• sed = but
• kaj = and

Here the speaker is making a contrast:

• we should not always change our habits,
• but sometimes a new habit is very necessary.

That contrast is exactly why sed is used. Kaj would just join two ideas without showing that contrast.