"Anne is an effective woman."
Translation:Ana este o femeie eficace.
Here's some help with the "effective" part of your question; the other part is said to be a mystery. "Effective" is a term we hear about so often these days in relation to another term, "efficient".
effective = producing the result that is wanted or intended; producing a successful result (https://www.oxfordlearnersdictionaries.com/definition/english/effective?q=effective)
efficient = doing something well and thoroughly with no waste of time, money or energy (https://www.oxfordlearnersdictionaries.com/definition/english/efficient?q=efficient)
The following article goes into more detail regarding the difference between the two: https://www.huffingtonpost.com/david-peck/efficient-or-effective-leader-ways_b_2606615.html
And here's a multimedia version: https://www.youtube.com/watch?v=gDrAeyTt4hg
"un" is the indefinite article for the singular form of masculine and neuter nouns.
"o" is the indefinite article for the singular form of feminine nouns.
un = a/an, o = a/an
Also, "un" and "o" can be used as numerals (one). Article or numeral - it depends on the context.
MASCULINE SG - PL: UN băiat - DOI băieți --- a/one boy - two boys
FEMININE SG - PL: O fată - DOUĂ fete --- a/one girl - two girls
NEUTER SG - PL: UN ou - DOUĂ ouă --- an/one egg - two eggs