Expressing “constantly” with 一直 and 一贯
We use these adverbs to emphasize that something or some action never changes and always stays the same.
The adverb 一贯
We use the adverb 一贯 
when we are talking about mindset, manners or attitudes toward something.
when we are talking about mindset, manners or attitudes toward something. Pay attention
We can not use the adverb 一贯 when we are talking about actions.
Examples
他听起来很诚恳，不过，话说回来，他一贯如此
He sounded sincere, but then again, he is always like that
tā tīngqǐlái hěn chéngkěn, búguò, huàshuō huílái, tā yíguàn rúcǐ
他一贯是个好老师
He was always a good teacher
tā yíguàn shì ge hǎo lǎoshī
The adjective 一贯
Sometimes we can use 一贯 as an adjective meaning “usual", "consistent”.
Pay attention
We can not use 一直 as an adjective.
as an adjective.Examples
这是他一贯的态度
This is his usual attitude
zhè shì tā yíguàn de tàidu
承认自己犯了错误可不是他一贯的作风
Admitting that he made a mistake is not his usual style
chéngrèn zìjǐ fàn le cuòwù kě bú shì tā yíguàn de zuòfēng
The adverb 一直
We use the adverb 一直 when we are talking about actions or states.
when we are talking about actions or states.Examples
雨一直下了三天
It rained constantly for three days
yǔ yìzhí xià le sān tiān
他一直是这个样子
He has always been like this
tā yìzhí shì zhè ge yàngzi
If there is another adverb before the predicate, we should state 一直 before that adverb.
Examples
从今天开始，我们一直互相帮忙
From today onwards, we will always help each other
cóng jīntiān kāishǐ, wǒmen yìzhí hùxiāng bāngmáng
Unlike 一贯, we can use 一直 when talking about the future.
Examples
要把这种作风一直坚持下去
You should always keep this attitude
yào bǎ zhè zhǒng zuòfēng yìzhí jiānchí xiàqù