An “and” operator for an “if” statement in Bash

I’m trying to create a simple Bash script to check if the website is down and for some reason the “and” operator doesn’t work:

#!/usr/bin/env bash

WEBSITE=domain.com
SUBJECT="$WEBSITE DOWN!"
EMAILID="an@email.com"
STATUS=$(curl -sI $WEBSITE | awk '/HTTP\/1.1/ { print $2 }')
STRING=$(curl -s $WEBSITE | grep -o "string_to_search")
VALUE="string_to_search"

if [ $STATUS -ne 200 ] && [[ "$STRING" != "$VALUE" ]]; then
    echo "Website: $WEBSITE is down, status code: '$STATUS' - $(date)" | mail -s "$SUBJECT" $EMAILID
fi

The “-a” operator also doesn’t work:

if [ $STATUS -ne 200 ] -a [[ "$STRING" != "$VALUE" ]]

Could you also please advise when to use:

• single and double square brackets
• parenthesis

?