Learn Hardcore Icelandic: Ég hef fylgt þessari áætlun í viku. - I have followed this schedule for a week.

It’s the present perfect (the Icelandic perfekt). You form it with the auxiliary hef (“have”) + the past participle fylgt (“followed”). It indicates that you began following the plan in the past and continue doing so (or that the result still matters now).

Why hef fylgt and not er að fylgja?

Hef fylgt expresses the perfect aspect (something done over a period up to now or completed with present relevance). Er að fylgja is the present continuous (“am following”) and highlights an action happening at this very moment. If you want both perfect and continuous you can say:

Ég hef verið að fylgja þessari áætlun í viku.

Why is þessi áætlun in the form þessari áætlun?

The verb fylgja requires its object in the dative case. Áætlun is a feminine noun, so the demonstrative þessi (this) takes the dative feminine singular ending, giving þessari áætlun.

Why is the preposition í used with viku, and what does it indicate?

When í is used with a time span in Icelandic, it denotes duration: í viku = “for a week.” Grammatically, you use í + the accusative (here viku—identical to the dative form) to express how long something lasts.

Could you use the simple past tense instead of the present perfect? How would that change the meaning?

Yes. You could say:

Ég fylgdi þessari áætlun í viku.
That’s the preterite (simple past). It states that you followed the plan for exactly one week in the past and implies you don’t follow it anymore. The perfect, by contrast, links the action to the present.

What case does the verb fylgja require, and why does that matter?

Fylgja (“to follow”) always takes a dative object. This is just one of Icelandic’s case-assigning verb properties. Because of that, any noun or pronoun following fylgja must be in the dative case—hence þessari áætlun rather than a nominative or accusative form.

How would you ask “Have you followed this plan for a month?” in Icelandic?

You invert the auxiliary and the subject and replace viku with mánuð:

Hefurðu fylgt þessari áætlun í mánuð?
(You can also say í einn mánuð to emphasize “one month.”)

How do you say “I have followed this plan since Monday” in Icelandic?

Use síðan + a point in time:

Ég hef fylgt þessari áætlun síðan mánudag.
(“Since Monday.”)

What’s the difference between í viku and um viku?

í viku = duration (“for one week”). E.g., Ég hef fylgt í viku = “I’ve been following for a week.”
um viku = a point in the future (“in a week”). E.g., Ég byrja um viku = “I start in a week.”

Why is there no definite article before viku when talking about durations?

In Icelandic, indefinite time spans don’t take the definite article. You say í viku (“for a week”) without k (“the week”). If you meant a specific week (like this week), you would use the demonstrative in the dative: í þessari viku (“in this week”).