Learn Polish: Codziennie pijemy półtora litra soku. - We drink a litre and a half of juice every day.

Półtora means "one and a half" and is used with masculine and neuter nouns. Since litr (liter) is a masculine noun, we use półtora. If we were counting a feminine noun, like godzina (hour), we would use półtorej instead.

Why is the word for liter spelled litra in this sentence?

Number words for fractions, like pół (half) and półtora (one and a half), force the noun that follows them to take the Genitive case. You can think of it as meaning "one and a half OF a liter". The Genitive singular form of litr is litra.

Why is sok (juice) also in the Genitive case (soku)?

We use the Genitive case here to express the English word "of". Whenever you have a measurement, container, or amount of something, the substance being measured goes into the Genitive. So litra soku literally translates to "a liter of juice".

Could we use a perfective verb for "drink" here instead of pijemy?

No, because the adverb codziennie (every day) tells us this is a repeated, routine action. In Polish, routine or habitual actions always use imperfective verbs like pijemy because they focus on the ongoing habit rather than a single completed result.