Vespere mi ĉiam ŝlosas la pordon, ĉar la malnova seruro ne funkcias bone.

Vespere is an adverb meaning in the evening. It comes from vespero (evening) plus the adverb ending -e.

Esperanto often uses a simple adverb of time where English uses a prepositional phrase. So:

• vespere = in the evening
• matene = in the morning
• nokte = at night

En la vespero is possible, but vespere is shorter and very natural.

By itself, vespere usually just means in the evening or during the evening.

In this sentence, the idea of repetition comes from ĉiam (always). So together, Vespere mi ĉiam... gives the sense of Every evening, I always... or more naturally I always ... in the evening.

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

Here, mi ĉiam ŝlosas means I always lock. That is a very normal placement.

You could also hear:

• Vespere ĉiam mi ŝlosas la pordon
• Mi ŝlosas la pordon ĉiam vespere

But Vespere mi ĉiam ŝlosas la pordon sounds natural and clear.

The ending -as marks the present tense in Esperanto.

But just like English present tense, it can describe:

• something happening now
• a habit
• something generally true

Here it is habitual: mi ĉiam ŝlosas = I always lock.

So -as works well for repeated everyday actions.

This is an important difference:

• fermi = to close
• ŝlosi = to lock

So:

• Mi fermas la pordon = I close the door
• Mi ŝlosas la pordon = I lock the door

A door can be closed without being locked, so Esperanto keeps those ideas separate.

The -n marks the direct object.

Here:

• mi = the subject, the person doing the action
• ŝlosas = the action
• la pordon = the thing being locked

So pordon gets -n because it is the direct object.

This is the accusative ending, one of the most important features of Esperanto grammar.

Because la never changes.

In Esperanto, the definite article is always la, no matter the number, gender, or case. The case ending goes on the noun, and also on any adjective modifying it, not on the article.

So:

• la pordo = the door
• la pordon = the door as a direct object

If there were an adjective, it would also take -n:

• la malnovan pordon

But la itself still stays la.

Malnova is built from:

• nova = new
• mal- = a prefix meaning the opposite

So:

• nova = new
• malnova = old

The prefix mal- is very common in Esperanto and is used to form opposites:

• bona = good → malbona = bad
• granda = big → malgranda = small
• fermi = close → malfermi = open

So malnova seruro literally means an old lock.

These words are related but different:

• seruro = lock (the mechanism in the door)
• ŝlosilo = key
• ŝlosi = to lock

So in this sentence:

• la malnova seruro = the old lock
• mi ŝlosas la pordon = I lock the door

A learner can easily mix up seruro and ŝlosilo, so this is a good pair to remember.

Ĉar means because and introduces a full clause.

So:

• ĉar la malnova seruro ne funkcias bone = because the old lock does not work well

This is different from pro, which is a preposition and is followed by a noun phrase, not a full clause.

Compare:

• Mi restis hejme, ĉar mi estis malsana.
• Mi restis hejme pro malsano.

In your sentence, there is a full subject + verb part after ĉar, so ĉar is the right choice.

Because bone is an adverb, and it modifies the verb funkcias.

• bona = good (adjective, describes a noun)
• bone = well (adverb, describes a verb)

Here the meaning is does not function well, so the adverb is needed:

• La seruro ne funkcias bone.

If you said bona, it would be grammatically wrong here, because funkcias needs an adverb, not an adjective.

Yes. Esperanto word order is fairly flexible, especially because the -n ending helps show what the object is.

For example, these are all possible with slightly different emphasis:

• Vespere mi ĉiam ŝlosas la pordon
• Mi ĉiam ŝlosas la pordon vespere
• La pordon mi ĉiam ŝlosas vespere

The original sentence begins with Vespere, which gives a natural time-setting: In the evening...

They are pronounced like this:

• ŝ = English sh as in ship
• ĉ = English ch as in church

So:

• ŝlosas sounds roughly like SHLO-sas
• ĉiam starts with a ch sound
• ĉar also starts with ch

Esperanto spelling is very regular, so each letter has one consistent sound.