Mi lavas miajn manojn per sapo.

In Esperanto, -as marks the present tense. So lavas means wash / am washing / do wash, depending on context.

A few useful comparisons:

• lavas = present
• lavis = past
• lavos = future
• lavu = imperative / jussive
• lavi = infinitive, to wash

So Mi lavas... means I wash or I am washing...

Because they are both describing the direct object, and Esperanto shows that very clearly.

Here is the breakdown:

• mano = hand
• manoj = hands (-j = plural)
• manojn = hands as a direct object (-n = accusative)

And the adjective/possessive must agree with the noun:

• mia = my
• miaj = my (plural things)
• miajn = my (plural direct object things)

So:

• miajn manojn = my hands as the thing being washed

Esperanto adjectives and possessives agree with the noun in:

• number: singular/plural
• case: whether it has -n

Because the hands are what is being washed. In Esperanto, the direct object usually takes -n.

In this sentence:

• Mi = the subject, the one doing the action
• lavas = wash
• miajn manojn = the direct object, the thing being washed

So the accusative ending -n marks manojn as the object of the verb.

A simple comparison:

• La manoj estas puraj. = The hands are clean.
  No direct object, so no -n.
• Mi lavas la manojn. = I wash the hands.
  manojn is the direct object, so it gets -n.

Because mia has to agree with manojn.

Since manojn is:

• plural → it needs -j
• accusative/direct object → it needs -n

the matching possessive form is:

• mia = my
• miaj = my, plural
• miajn = my, plural and accusative

So miajn manojn is the correct fully matching form.

Yes, you could say mian manon if you mean one hand.

The sentence uses miajn manojn because people normally wash both hands, so the plural is natural.

Compare:

• Mi lavas mian manon. = I wash my hand.
• Mi lavas miajn manojn. = I wash my hands.

Both are grammatically correct; they just mean different things.

Per means with, by means of, using. It shows the instrument or means used to do something.

So:

• per sapo = with soap / using soap

This is different from English with, which can mean several things. In Esperanto, per is specifically about the means or tool.

Examples:

• Mi skribas per krajono. = I write with a pencil.
• Li malfermis la pordon per ŝlosilo. = He opened the door with a key.

So in your sentence, soap is the means used for washing.

Because sapo is not the direct object. You are not washing the soap; you are washing your hands.

The roles are:

• miajn manojn = direct object → gets -n
• per sapo = prepositional phrase → no accusative here

In Esperanto, nouns after a preposition usually do not take -n, unless there is a special reason involving direction or measure. Here, per sapo is just a normal prepositional phrase, so sapo stays in its basic form.

Esperanto does have a definite article: la. But it is often omitted when English would naturally use a possessive instead.

In English, we say:

• I wash my hands

not usually:

• I wash the my hands

Likewise in Esperanto, miajn already tells you whose hands they are, so la is unnecessary.

You could also say:

• Mi lavas la manojn. = I wash the hands / I wash hands

But miajn manojn is more specific: my hands.

Yes, Esperanto word order is fairly flexible, because the endings show the grammatical roles.

The most neutral order is:

• Mi lavas miajn manojn per sapo.

But other orders are possible, for example:

• Per sapo mi lavas miajn manojn.
• Miajn manojn mi lavas per sapo.

These can change the focus or emphasis, but the meaning stays basically the same because manojn is still marked as the object.

That said, the original order is the most straightforward and natural for learners.

It should be miajn manojn.

In Esperanto, sia / si is a third-person reflexive form. It is used when the subject is li, ŝi, ĝi, ili, or a noun, not usually with mi or vi.

So:

• Mi lavas miajn manojn. = correct
• Vi lavas viajn manojn. = correct
• Li lavas siajn manojn. = correct, he washes his own hands

But:

• Mi lavas siajn manojn. = not correct in normal Esperanto

So for I, you use miajn, not siajn.

The dictionary forms are:

• mi = I
• lavi = to wash
• mia = my
• mano = hand
• per = with, by means of
• sapo = soap

Then the sentence builds from those forms using regular endings:

• lavi → lavas
• mano → manoj → manojn
• mia → miajn

This is one of the nice things about Esperanto: once you know the endings, it is easy to recognize how words are functioning in the sentence.