(Seriously though, reading the manpage for man is where I first learned about apropos.)

Google "linux how much space on drive" and the first hit tells you exactly what you need.

Another question someone might ask is "how do I change my password". Google for "linux change password" and there is a special help box right at the top of the search results.

apropos password and you get this enormous list:

  apg (1)              - generates several random passwords
  chage (1)            - change user password expiry information
  chgpasswd (8)        - update group passwords in batch mode
  chpasswd (8)         - update passwords in batch mode
  cpgr (8)             - copy with locking the given file to the password or gr...
  cppw (8)             - copy with locking the given file to the password or gr...
  cracklib-check (8)   - Check passwords using libcrack2
  create-cracklib-dict (8) - Check passwords using libcrack2
  crypt (3)            - password and data encryption
  crypt_r (3)          - password and data encryption
  des_read_2passwords (3ssl) - Compatibility user interface functions
  des_read_password (3ssl) - Compatibility user interface functions
  doveadm-pw (1)       - Dovecot's password hash generator
  endpwent (3)         - get password file entry
  endspent (3)         - get shadow password file entry
  EVP_BytesToKey (3ssl) - password based encryption routine
  expiry (1)           - check and enforce password expiration policy
  fgetpwent (3)        - get password file entry
  fgetspent (3)        - get shadow password file entry
  fgetspent_r (3)      - get shadow password file entry
  getpass (3)          - get a password
  getpw (3)            - reconstruct password line entry
  getpwent (3)         - get password file entry
  getpwnam (3)         - get password file entry
  getpwnam_r (3)       - get password file entry
  getpwuid (3)         - get password file entry
  getpwuid_r (3)       - get password file entry
  getspent (3)         - get shadow password file entry
  getspent_r (3)       - get shadow password file entry
  getspnam (3)         - get shadow password file entry
  getspnam_r (3)       - get shadow password file entry
  git-credential-cache (1) - Helper to temporarily store passwords in memory
  gitcredentials (7)   - providing usernames and passwords to Git
  grpconv (8)          - convert to and from shadow passwords and groups
  grpunconv (8)        - convert to and from shadow passwords and groups
  grub-mkpasswd-pbkdf2 (1) - generate hashed password for GRUB
  lckpwdf (3)          - get shadow password file entry
  login.defs (5)       - shadow password suite configuration
  lppasswd (1)         - add, change, or delete digest passwords.
  Net::LDAP::Control::PasswordPolicy (3pm) - LDAPv3 Password Policy control object
  Net::LDAP::Extension::SetPassword (3pm) - LDAPv3 Modify Password extension ob...
  pam_pwhistory (8)    - PAM module to remember last passwords
  pam_unix (8)         - Module for traditional password authentication
  passwd (1)           - change user password
  passwd (1ssl)        - compute password hashes
  passwd (5)           - the password file
  passwd2des (3)       - RFS password encryption
  putpwent (3)         - write a password file entry
  putspent (3)         - get shadow password file entry
  pwck (8)             - verify integrity of password files
  pwconv (8)           - convert to and from shadow passwords and groups
  pwunconv (8)         - convert to and from shadow passwords and groups
  seahorse (1)         - Passwords and Keys
  setpwent (3)         - get password file entry
  setspent (3)         - get shadow password file entry
  sgetspent (3)        - get shadow password file entry
  sgetspent_r (3)      - get shadow password file entry
  shadow (5)           - shadowed password file
  shadowconfig (8)     - toggle shadow passwords on and off
  smbpasswd (5)        - The Samba encrypted password file
  smbpasswd (8)        - change a user's SMB password
  ulckpwdf (3)         - get shadow password file entry
  unix_chkpwd (8)      - Helper binary that verifies the password of the curren...
  unix_update (8)      - Helper binary that updates the password of a given user
  vigr (8)             - edit the password, group, shadow-password or shadow-gr...
  vipw (8)             - edit the password, group, shadow-password or shadow-gr...
  xcrypt (3)           - RFS password encryption
  xdecrypt (3)         - RFS password encryption
  xencrypt (3)         - RFS password encryption

    $ apropos -a change password
    chage (1)            - change user password expiry information
    kpasswd (1)          - change a user's Kerberos password
    passwd (1)           - change user password
    PKCS12_newpass (3ssl) - change the password of a PKCS12 structure

    $ apropos -a space disk
    df (1)               - report file system disk space usage
    df (1p)              - report free disk space

man -k -S1 password

I use man and apropos all of the time, but I have no illusion that they're new user friendly. They're amazing when you just need to look up some options or get the syntax for a system call, but if you've not assimilated the Unix way of thinking they're quite obtuse.