Honesta amiko diras la veron, kaj pro tia honesteco oni povas fidi lin aŭ ŝin.

Because honesta amiko is the subject of the sentence: the honest friend is the one doing the action.

In Esperanto:

• adjectives end in -a
• nouns end in -o
• adjectives agree with the nouns they describe in number and case

Here both words are singular and in the normal subject form, so we get:

• honesta
• amiko

If it were a direct object, both would take -n:

• Mi vidas honestan amikon. = I see an honest friend.

The -n marks the direct object. In this sentence, the thing being told is the truth, so la veron is the object of diras.

• diras = says / tells
• la veron = the truth

So:

• Honesta amiko diras la veron. = An honest friend tells the truth.

This -n ending is one of the most important grammar markers in Esperanto.

Because diri la veron is the normal way to say tell the truth.

The word la here does not mean a very specific truth that was mentioned earlier. It is often used in Esperanto for general, understood ideas, and la vero works much like English the truth.

So:

• diri la veron = to tell the truth

Without la, the phrase would sound less standard here.

diras is the present tense form of diri.

Esperanto verb endings are very regular:

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

So:

• diri = to say
• diras = says / is saying

Also, Esperanto verbs do not change for person:

• mi diras
• vi diras
• li diras
• ŝi diras
• oni diras

All use diras.

Pro means because of, due to, or on account of.

So:

• pro tia honesteco = because of such honesty

It explains the reason for the next idea:

• because of that honesty, people can trust the friend

Also notice that after a preposition like pro, you normally do not add -n just because it comes after the preposition.

Because tia means such, or of that kind, while tiu means that one.

Here the sentence is talking about a kind or quality of honesty:

• tia honesteco = such honesty / that kind of honesty

If you used tiu, it would point more directly to a specific honesty as a particular thing:

• tiu honesteco = that honesty

In this sentence, tia is the more natural choice because the idea is honesty of that sort.

Honesteco is made from:

• honest- = honest
• -ec- = the quality or state of being something
• -o = noun ending

So:

• honesta = honest
• honesteco = honesty

This suffix -ec- is very useful in Esperanto:

• bela = beautiful → beleco = beauty
• granda = big → grandeco = greatness / size
• afabla = kind → afableco = kindness

Oni is an impersonal pronoun meaning something like:

• one
• people
• someone in general
• sometimes English you in a general sense

So:

• oni povas fidi... = one can trust... / people can trust...

It does not refer to a specific person. It makes the statement general.

Because povas is the conjugated verb, and fidi stays in the infinitive after it.

This is a very common Esperanto pattern:

• povas fidi = can trust
• volas iri = wants to go
• devas lerni = must learn

Only the first verb gets the tense ending:

• oni povas fidi = one can trust
• oni povis fidi = one could trust
• oni povos fidi = one will be able to trust

Because fidi is taking a direct object here, so the pronouns are in the object form:

• li = he
• lin = him
• ŝi = she
• ŝin = her

So:

• fidi lin aŭ ŝin = to trust him or her

The phrase refers back to amiko. Since amiko does not itself tell you whether the friend is male or female, the sentence spells out both possibilities: lin aŭ ŝin.

If the sex were known, you would normally use just one:

• oni povas fidi lin
• oni povas fidi ŝin

This word order is the most neutral and natural one, but Esperanto word order is somewhat flexible because endings show the grammar.

Here the basic order is:

• Honesta amiko = subject
• diras = verb
• la veron = object

Because veron has -n, you can still recognize it as the object even if the order changes. But for learners, the standard order used here is the best one to copy.

So yes, the sentence could be rearranged in some contexts, but this version is the clearest and most ordinary.