should be because of this:
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ohhh i havent reached that grammer point yet. i shall look at it now. thank you 
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In 〜ようと思う、〜よう requires the main verb to be conjugated properly to make a volitional form.
You can find an expanded conjugation table here: Japanese Verb Volitional Form よう.
Godan
会う → 会おう
立つ → 立とう
写す → 写そう
割る → 割ろう
書く → 書こう
泳ぐ → 泳ごう
死ぬ → 死のう
学ぶ → 学ぼう
休む → 休もう
Ichidan
食べる → 食べよう
起きる → 起きよう
閉じる → 閉じよう
Irregular
来る → 来よう
する → しよう
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Hi @Folklorien, the reason this is 乗り換え instead of 乗り換える is because of the よう afterward, which is an auxiliary verb that attaches to the stem form of verbs.
For the vocabulary decks, particularly い-Adjectives and verbs, we only ever require that the user enter either the full word, or the stem form (we’d never ask you to type 乗り換えよう or 乗り換えたい for example). This is so that memorizing grammar patterns isn’t required when focusing strictly on vocabulary study. Due to this, if the full word isn’t accepted in any review, you can assume in 99% of cases that something immediately after the word is causing that, and the stem form will be needed instead.
While you don’t need to ‘know’ the grammar to fill in the blanks in vocab, it will certainly help, so my advice would be to take note of anything that comes after the vocab you are studying that appears new to you. Hope this helps! 
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