Learn Spanish (Spain): Este año hemos conseguido el objetivo. - This year we have achieved the objective.

The phrase este año (this year) refers to an unfinished timeframe. Because the year isn't over yet, Spanish speakers in Spain use the Pretérito Perfecto (hemos conseguido) to talk about what happened during this time.

How would this sentence change if we were talking about "last year" instead?

If you change it to el año pasado (last year), the time period is completely finished. For finished timeframes, you must switch to the Pretérito Indefinido and say: El año pasado conseguimos el objetivo.

What is the infinitive (dictionary form) of conseguido?

It comes from conseguir, which means to achieve, get, or obtain. To form the past participle for an -ir verb like this one, you just drop the ending and add -ido, giving you conseguido.

Could I put words in the middle of the verb, like "hemos ya conseguido"?

No, in Spanish you can never separate the two parts of this past tense. The helping verb (hemos) and the past participle (conseguido) must always stay glued together. Any extra words, like ya (already), need to go before or after the entire verb block.