Við þurfum að breyta áætluninni.

Þurfa is the dictionary form, meaning to need. In the sentence, the verb has to match við (we), so it changes form.

Present tense of þurfa:

• ég þarf — I need
• þú þarft — you need
• hann/hún/það þarf — he/she/it needs
• við þurfum — we need
• þið þurfið — you plural need
• þeir/þær/þau þurfa — they need

So við þurfum means we need.

Here að is the infinitive marker, like English to in to change.

So:

• breyta = change
• að breyta = to change

In Icelandic, after many verbs such as þurfa (need), ætla (intend), vilja (want), and others, you often get að + infinitive.

So:

• Við þurfum að breyta ... = We need to change ...

Because breyta takes the dative case, not the accusative.

This is one of the most important things to learn in Icelandic: many verbs require a specific case for the noun that follows them. Breyta means to change / alter, and the thing being changed is put in the dative.

So:

• base noun: áætlun — plan
• definite form in the dative singular: áætluninni — the plan

That is why the sentence uses áætluninni.

A useful pattern to remember:

• breyta einhverju = change something
  where einhverju is dative

The ending -inni shows two things at once:

1. the noun is definite — the plan, not just a plan
2. the noun is in the dative singular

The noun áætlun is a feminine noun. Its forms include:

• áætlun — a plan (nominative singular)
• áætlunina — the plan (accusative singular)
• áætluninni — the plan (dative singular)
• áætlunarinnar — of the plan (genitive singular)

So -inni here is not random; it is the normal dative definite ending for this noun.

Við is in the nominative, because it is the subject of the sentence.

In this sentence:

• Við = subject = we
• þurfum = verb = need
• að breyta áætluninni = infinitive phrase = to change the plan

So the overall structure is:

• Við — nominative subject
• þurfum — finite verb
• að breyta — infinitive
• áætluninni — dative object of breyta

Yes, breyta is a very common verb for change, especially when you mean alter, modify, or change something into a different form.

A very important pattern is:

• breyta einhverju — change something

Examples:

• breyta áætluninni — change the plan
• breyta reglunum — change the rules
• breyta þessu — change this

But Icelandic also has other expressions depending on meaning. For example:

• skipta um can mean change/switch in the sense of replacing something, as in clothes, buses, channels, etc.

So breyta is a good choice when you mean modify the plan, not necessarily replace it completely.

In Icelandic, the definite article is usually a suffix, attached to the end of the noun.

So instead of a separate word like English the, Icelandic often does this:

• áætlun — plan
• áætlunin — the plan

Then the ending changes depending on case:

• áætlunin — the plan (nominative)
• áætlunina — the plan (accusative)
• áætluninni — the plan (dative)

This is completely normal in Icelandic and is one of the first big differences English speakers notice.

A rough pronunciation guide for English speakers is:

Við þurfum að breyta áætluninni
≈ Vith THUR-vum ath BRAY-ta OW-eyeht-lu-nin-nih

A few notes:

• ð in Við is like the th in this
• þ in þurfum is like the th in thing
• ey in breyta sounds roughly like ay
• æ in áætluninni is often pronounced like eye
• stress usually falls on the first syllable of each word

If you want a simpler approximation:

• Við ≈ vith
• þurfum ≈ THUR-vum
• að ≈ ath
• breyta ≈ BRAY-ta

You can, but it would not mean exactly the same thing.

• Við þurfum að breyta áætluninni = We need to change the plan
• Við þurfum að breyta áætlun = We need to change a plan / plans / planning depending on context, though this sounds less specific

Also, because breyta takes the dative, the indefinite singular would normally be:

• Við þurfum að breyta áætlun
  Here the dative singular of some feminine nouns can look the same as the nominative/accusative form, so the case is not always obvious just from the ending.

In your sentence, the article makes the form very clear: áætluninni = the plan in the dative.

The sentence has a very normal word order:

• Við — subject
• þurfum — verb
• að breyta áætluninni — infinitive phrase

So it is basically: We need to change the plan.

But Icelandic has verb-second word order in main clauses, which means the finite verb often stays in the second position. Because of that, other word orders are possible for emphasis or style.

For example:

• Nú þurfum við að breyta áætluninni. — Now we need to change the plan.

Here nú comes first, and the verb þurfum still stays in second position, before við.

So the sentence you have is the basic, straightforward order, but Icelandic can move things around more than English.

Here is a breakdown:

• Við — we, nominative subject
• þurfum — need, 1st person plural present of þurfa
• að — infinitive marker, to
• breyta — change, infinitive
• áætluninni — the plan, dative singular definite of áætlun

So the grammar is:

subject + finite verb + infinitive marker + infinitive + dative object

A compact gloss would be:

• Við = we
• þurfum = need
• að breyta = to change
• áætluninni = the plan (dative)

This is a very useful model sentence because it shows:

• verb agreement
• að + infinitive
• a verb that governs the dative
• a noun with the suffixed definite article