The word concentrate in my English Esperanto dictionary is Koncentr(igx)i.
I thought that the suffix igx changed a word from transitive to intransitive. Isn’t the word concentrate already intransitive. What then does igx do?

Concentrate indeed is what you do at exams (and intransitive), but it is also what you do to solutions to make them concentrated (transitive)! This is a problem with translating dictionaries - they can be ambiguous, because you don't know which meaning of the english word is being used. The best thing is to check a "regular" dictionary as well.

The Reta Vortaro says:

Koncentri: (tr.) 1. Kunigi dense ĉirkaŭ komuna centro; alcentrigi koncentri trupojn, ŝipojn, kaptitojn; (figure) koncentri sian atenton; (figure) ni koncentru niajn klopodojn kaj penadojn sur la junularon. 2. Pliigi la proporcion de solvita substanco en solvaĵo per relativa forigo de la fluaĵoj.

So koncentri is transitive, and corresponds to what we mean when we say "they are concentrating troops in preparation for war" or "a concentrated solution".

Reta Vortaro also says:

koncentriĝi: 1. Iĝi koncentrita. 2. (vidu "koncentrigi sin") Streĉi la intelektajn fortojn super solvota problemo.

So to "koncentriĝi" is to "stretch your intellectual strength over a to-be-solved problem" which sounds like what we do at exams .

nshepperd:

Your answer makes a lot of sense. I was thinking of the word only as intransitive - I cannot concentrate!

Dankon,
Ailebol

Senegaùlo:I think concentrate can never be intransitive. "I concentrate" means "I concentrate myself = I make myself concentrated...

"concentrate" (the English word) can be transitive and intransitive. English is loosey-goosey that way.

"koncentri" (the Esperanto word), on the other hand...

I.e. don't confuse the English word "concentrate" with the Esperanto word "koncentri". Statements about one aren't necessarily valid about the other.

"Learning" a word only by learning a one-word English equivalent often leads to confusion. One needs to learn the meaning of a word, not just an English one-word equivalent.

E.g. when people simply learn lists of eo-en word pairs like this:
droni = drown
fini = finish
fleksi = bend
komenci = begin
ligi = link
ruli = roll
they are bound to make errors in Esperanto, since all those English words have at least 2 meanings (due to loosey-goosy transitivity in English), not all of which are shared by the Esperanto words.

You need to learn that (for example) "droni" means "to die in water", not just that it is the same as "drown", since "drown" is a multi-meaning English word - a subject can drown, or a subject can drown an object - 2 quite different experiences for the subject!

Otherwise you will make mistakes by expressing "He drowned the cat" as *"Li dronis la katon"*, which is wrong and should be "Li dronigis la katon". (And so "La kato dronis.")