Mi helpis la sinjorinon.

Because -is is the Esperanto ending for the past tense.

• helpi = to help
• helpas = helps / is helping
• helpis = helped
• helpos = will help

So Mi helpis la sinjorinon means that the helping happened in the past.

Mi means I.

Unlike English, Esperanto pronouns do not change for subject position the way English does with I / me. The form mi stays the same.

Examples:

• Mi helpis ŝin = I helped her
• Ŝi helpis min = She helped me

Here, mi is the subject, so it means I.

The -n ending marks the direct object.

In this sentence:

• Mi = the person doing the action
• helpis = did the helping
• la sinjorinon = the person receiving the action

So sinjorino becomes sinjorinon because she is the one being helped.

This is called the accusative ending in Esperanto.

Because in Esperanto, the -n goes on the noun and on any words that agree with it, not on the article la.

So:

• la = the
• sinjorino = lady / woman / madam
• la sinjorinon = the lady, as a direct object

The article la never changes. It is always just la.

La is the Esperanto definite article, meaning the.

So:

• sinjorino = a lady / lady
• la sinjorino = the lady

Esperanto has only one definite article: la.
It does not change for gender, number, or case.

Yes. That would usually mean I helped a lady or I helped some lady, rather than the lady.

So the difference is:

• Mi helpis la sinjorinon = I helped the lady
• Mi helpis sinjorinon = I helped a lady

Esperanto has no special word for a/an, so leaving out la often gives that indefinite sense.

Usually, yes. Helpi commonly takes a direct object in Esperanto.

So:

• Mi helpis la sinjorinon = I helped the lady
• Li helpas min = He helps me

This is one place where Esperanto is a bit more straightforward than English, because you simply mark the person being helped as the direct object with -n.

Yes. Esperanto word order is fairly flexible because the -n ending shows what the direct object is.

So all of these are possible:

• Mi helpis la sinjorinon
• La sinjorinon mi helpis

Both still mean that I helped the lady, because sinjorinon has the object ending -n.

However, Mi helpis la sinjorinon is the most neutral and basic order.

Sinjorino means lady, madam, or sometimes Mrs. depending on context.

It is the feminine form related to sinjoro:

• sinjoro = gentleman, sir, Mr.
• sinjorino = lady, madam, Mrs.

In your sentence, it simply refers to a woman politely or respectfully.

You would add -j for the plural, and because it is still the direct object, you would also keep -n.

So:

• la sinjorino = the lady
• la sinjorinoj = the ladies
• la sinjorinojn = the ladies, as a direct object

Example:

• Mi helpis la sinjorinojn = I helped the ladies

In Esperanto, plural -j comes before accusative -n.

It is pronounced roughly like:

seen-yo-REE-non

A few helpful points:

• sj sounds like a soft sy sound
• j in Esperanto sounds like English y
• the stress is on the second-to-last syllable

So in sinjorinon, the stress falls on ri: sin-jo-RI-non

No. The grammar clearly shows that I am the subject and the lady is the object.

• Mi = I
• la sinjorinon = the lady, marked as the direct object

If you wanted to say The lady helped me, you would say:

La sinjorino helpis min.

Here, min has the -n ending, so me is the object.