Mia tornistro enhavas tendon.

Because tendon is the direct object of the verb enhavas.

In Esperanto, the direct object usually takes the accusative ending -n.
Here:

• Mia tornistro = the subject
• enhavas = contains
• tendon = the thing being contained, so it gets -n

So:

• tendo = a tent
• tendon = a tent as the direct object

Because tornistro is the subject, not the object.

The backpack is doing the action of containing, so it stays in the basic noun form:

• tornistro = backpack

Only the direct object gets -n here:

• tendon = tent

The ending -as marks the present tense in Esperanto.

So enhavas means contains or is containing.

A useful pattern:

• -as = present
• -is = past
• -os = future
• -us = conditional
• -u = command / wish

So enhavas is present tense because the sentence is talking about what the backpack contains now.

Havas means has, while enhavas means contains.

So:

• Mi havas tendon = I have a tent.
• Mia tornistro enhavas tendon = My backpack contains a tent.

The prefix en- often has the idea of in or inside, so enhavi gives the sense of have inside.

Because mia tornistro is the subject of the sentence.

In Esperanto, adjectives and possessives agree with the noun they describe. If the noun had -n, the possessive would also take -n:

• mia tornistro = my backpack
• mian tornistron = my backpack as a direct object

But in this sentence, mia tornistro is the subject, so neither word takes -n.

Esperanto does not have an indefinite article like English a/an.

It does have a definite article, la, for the, but it is only used when needed.

So:

• tornistro can mean a backpack or just backpack depending on context
• tendo can mean a tent
• la tornistro would mean the backpack

In Mia tornistro enhavas tendon, there is no la because the sentence does not specifically say the tent.

Yes, to some extent.

Because tendon has the accusative -n, Esperanto allows more flexible word order than English. For example, these mean essentially the same thing:

• Mia tornistro enhavas tendon.
• Tendon enhavas mia tornistro.

The -n makes it clear that tendon is still the object.

That said, Mia tornistro enhavas tendon is the most straightforward and natural order for a learner: subject + verb + object.

Because it does not have the plural ending -j.

In Esperanto:

• singular noun: tornistro
• plural noun: tornistroj

So:

• mia tornistro = my backpack
• miaj tornistroj = my backpacks

The same applies to tendo:

• tendo = tent
• tendoj = tents

You would add -j to the plural nouns, and adjectives/possessives would agree with them.

For example:

• Miaj tornistroj enhavas tendojn. = My backpacks contain tents.

Breakdown:

• miaj = my, plural
• tornistroj = backpacks
• enhavas = contain / contains
• tendojn = tents as a direct object, so both -j and -n

This is a good example of Esperanto agreement:

• plural: -j
• object: -n
• both together: -jn

A simple approximate pronunciation is:

MEE-ah tor-NEE-stro en-HAH-vas TEN-don

A few helpful notes:

• mia has two clear vowels: mi-a
• o is always pronounced clearly, like a pure vowel
• e is always a pure e
• stress in Esperanto is usually on the second-to-last syllable

So:

• Mia → stress on MI
• tornistro → stress on NIS
• enhavas → stress on HA
• tendon → stress on TEN

Yes.

The dictionary forms are:

• mia = my
• tornistro = backpack / knapsack
• enhavi = to contain
• tendo = tent

In the sentence, two of them are inflected:

• enhavi becomes enhavas because it is present tense
• tendo becomes tendon because it is the direct object

This is very typical in Esperanto: once you know the endings, you can often work backward to the base form easily.