Learn Finnish: Optikko tutki molemmat silmät. - The optometrist examined both eyes.

In Finnish, complete actions take a total object. Because the optometrist finished examining the entirety of both eyes, the object takes the plural accusative case. For all plural words, the accusative looks exactly like the basic nominative plural, ending in -t.

What if the examination was still ongoing? How would molemmat change?

If the action was incomplete, the object would need to be in the partitive plural to show an ongoing process. The sentence would become Optikko tutki molempia silmiä. Notice that the stem of molemmat changes to the strong grade molemp- when taking the partitive plural ending -ia.

Why doesn't the past tense verb tutki have a noticeable past tense marker?

The verb tutkia has a stem that already ends in -i (tutki-). To form the past tense, you would normally add the past marker -i, but Finnish merges the double -i- into a single -i. So, while the present tense is hän tutkii with a long vowel, the past tense is just hän tutki.

Can molemmat ever be used in singular cases?