Learn Finnish: Tämä retki ei ole pitkä.

Negative verbs usually force the object of a sentence into the partitive case. However, pitkä is not an object here; it is an adjective describing the subject (tämä retki). Because a trip is a single, countable thing, the adjective stays in the basic nominative case even when the sentence is negative.

How does the word retki behave when we need to add endings to it?

Retki is a classic Finnish word ending in -i. For these words, the -i changes to an -e- before you add most grammatical endings. For example, the stem is retke-, so the plural 'trips' becomes retket and 'on the trip' becomes retkellä.

What is the difference between retki and matka since both can mean 'trip'?

A retki is usually a shorter outing, excursion, or field trip, like a hike or a picnic. A matka generally refers to a longer journey or travel, such as a vacation to another country.

How do Finns usually say ei ole in everyday spoken language?

In casual spoken Finnish, ei ole is very frequently shortened to ei oo.